Mammalian body growth is rapid in early postnatal life, but then the growth rate slows, eventually declining toward zero as the organism approaches its adult body size. This decline in growth rate, which is due primarily to a decrease in the rate of proliferation, occurs simultaneously in multiple tissues but does not appear to be orchestrated by a hormonal or other systemic mechanism. We therefore hypothesized that this coordinated growth deceleration is directed by a program of gene expression that is common to multiple tissues. To test this hypothesis, we performed microarray analysis to identify changes in gene expression that occurred during early postnatal life in mice, as body growth slows. We particularly focused on genes that were up- or downregulated in multiple organs and thus more likely to contribute to the putative common program of growth deceleration. In each of the organs studied, kidney, lung, and heart, more than 2000 genes were found to be significantly upregulated with age and more than 2800 genes downregulated. The number of genes that were regulated coordinately in all three organs was strikingly high, with 1207 genes downregulated in all three organs and 428 upregulated in all three organs, far more overlap than would be expected by chance, indicating that there is an extensive program of gene expression common to multiple organs during postnatal growth, in addition to the expected tissue-specific changes. The common program included genes involved in regulating G1/S and G2/M checkpoints, Hedgehog signaling, and Wnt/beta-catenin signaling. There were more genes that were downregulated with age than were upregulated in these pathways. We next focused our attention on 5 genes, Igf2, Mest, Peg3, Sox4, and Igf2bp3, that were markedly downregulated in all three organs for more detailed characterization. First, we measured mRNA expression levels by real-time PCR in the same three organs studied by microarray. The real-time PCR results confirmed the marked declines in mRNA expression with age. We also measured mRNA levels in a fourth organ, liver, and found dramatic declines with age, similar in pattern to the declines found in heart, kidney, and lung, further confirming that this gene expression program is occurring globally in multiple organs. We next used in situ hybridization to determine which cell types in each organ expressed Igf2, Mest, and Peg3 in the 1-week-old mouse. In general, we found expression in organ-specific parenchymal cells. These findings suggest that the coordinate decline in expression in multiple organs is not occurring simply because gene expression is restricted to cells that are common to multiple organs, such as endothelial cells or stromal fibroblasts. Finally, we sought to determine whether the observed changes in gene expression are a function of time per se or a function of growth. In rats, propylthiouracil-induced hypothyroidism and growth retardation during the first 5 weeks of life delayed the declines in expression of Igf2, Mest, and Peg3, the three genes which we studied that have been implicated in growth regulation. These data are consistent with the hypothesis that the normal decline in expression of these genes is driven, not by age per se, but rather by the process of growth, and therefore, a prior period of growth inhibition slows this decline. Based on these findings we propose the following model to explain the deceleration and eventual cessation of somatic growth in mammals. In late embryonic and early postnatal life, a network of growth-promoting genes is expressed at high levels. The resulting growth causes the expression of these growth-promoting genes to decline, which in turn slows the rate of growth. Eventually, the expression levels of these growth-promoting genes declines sufficiently to cause somatic growth to cease. If some external condition, such as hypothyroidism transiently restricts growth, then the slow growth will slow the decline in gene expression, thus preserving future growth capacity. Following the period of growth inhibition, the expression levels of the growth-promoting genes will be higher than normal, and consequently the growth rate will be greater than normal. Thus the model provides an explanation for the phenomenon of catch-up growth which is defined as a growth rate that is greater than normal for age following a period of growth inhibition. Our findings indicate that there exists an extensive genetic program occurring during the postnatal period involving upregulation and downregulation of thousands of genes. Of these, some appear to be organ-specific, but many are regulated in a concerted fashion in multiple organs. Some of these common genes regulate cell proliferation and thus may constitute part of the mechanism that causes somatic growth to slow and eventually cease. We found evidence that several of the genes likely to participate in this growth-limiting program, Igf2, Mest, and Peg3, are themselves regulated by growth, suggesting that, in the embryo, a gene expression pattern is established that allows for rapid somatic growth of multiple tissues but then, during postnatal life, this growth leads to negative-feedback changes in gene expression that in turn slow and eventually halt somatic growth, thus imposing a fundamental limit on adult body size.