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SUMMARY Primary tissue cultures of adrenal cells were prepared from foetal rat adrenals. The effect of cyclic AMP (75 μmol/l) on the morphology and steroid synthetic activity of the cortical cells was examined in order to determine whether cyclic AMP mimics the effects induced by adrenocorticotrophin (ACTH); namely transformation to the differentiated cell type and increase in steroid synthetic activity. Cyclic AMP was found to induce some, but not all, of the changes normally induced by ACTH. In particular, the mitochondria in the cortical cells developed vesicular cristae and there was a proliferation of the smooth endoplasmic reticulum. These two organelles are the principle sites of enzymes involved in steroid synthesis and the ultrastructural transformation was accompanied by an increase in the steroidogenic activity of the cells. However, clear differences in the ultrastructure of the cyclic AMP- and ACTH-treated cells were noted. The effects of cyclic AMP on the ultrastructure of cortical and fibroblastic cells present in the adrenal cultures were found to be dependent upon the cell type.