The mammalian adrenal cortex is composed of three concentric zones of differentiated cells which perform distinct functions and which respond to a different set of regulators. The developmental nature of this zonation is not understood. Currently, the major thrust has been to separate the individual zones and examine their constituent cells in terms of structural characteristics and functional activities. The guinea pig, a cortisol producer like the human being, is utilized as an animal model. The following summarizes some of the findings: 1. Cells were examined by light and electron microscopy. Striking differences in neutral fat content and smooth endoplasmic reticulum were noted. Chronic dexamethasone suppression caused atrophy of the zona fasciculata and loss of neutral fat but had no affect on the zona reticularis. 2. The rate-limiting step in steroidogenesis is mitochondrial cholesterol side-chain cleavage (SCC). Acute stress and ACTH administration caused an increase while dexamethasone suppression caused a decrease in cholesterol SCC activity in the fasciculata but not the reticularis. 3. The content of unconjugated and sulfoconjugated steroids was determined in the two zones. In general, steroids with a double bond in ring A of the steroid nucleus were present in a higher concentration in the fasciculata while steroids with a double bond in ring B (unconjugated and sulfoconjugated) were more concentrated in the reticularis. 4. The content of ascorbic acid (AA), measured after HPLC isolation, was twice as high in the reticularis as in the fasciculata. However, chronic dexamethasone suppression caused depletion of AA only from the fasciculata. 5. Cells isolated from the fasciculata increased steroid production in response to ACTH and cAMP while reticularis cells did not respond. Cells from both the fasciculata and reticularis increased cAMP production in response to ACTH. Conclusion: the ACTH-insensitivity of the reticularis appears to be due to a defect beyond the formation of cAMP.