There is an apparnt incease in mitochondria of the renoprival kidney within 24 hours post-mononephrectomy. Increases in mitochondrial DNA, RNA and amino acid incorporation are observed in this period. Two mitochondrial populations (designated M1 and M2) have been isolated by isopycnic sucrose density gradient centrifugation from kidney. M1 appears to predominate in the cortex and M2 in the medulla; both are present in both sections as determined from incorporation studies. Half-lives of the two populations indicates that there is no precursor-product relationship between M1 and M2. There are marked biochemical differences in the two populations, suggesting they are independent. Electron microscopic evaluation of the populations is being conducted. The sequence of events leading to a new mitochondrion are being investigated. An early stimulation (3 hours) occurs in mitochondrial RNA synthesis with a peak at 12 hours; the new RNA synthesized at 12 hours does not appear to be as sensitive to actinomycin D inhibition as other cellular RNA's. Results suggest that part of the increase in mitochondrial content is due to a decreased rate of degradation. Studies are underway on determination of the time and site of synthesis of specific mitochondrial proteins. Changes in function of mitochondria from the renoprival kidneys are being investigated.