Work with the calcium-dependent cGMP phosphodiesterase is currently focused on mechanisms for regulation of calmodulin activity. Cyclic nucleotide phosphodiesterases in several tissues have been studied to characterize their regulatory properties. As part of a continuing investigation of the effects of glucocorticoids, we are comparing phosphodiesterases in liver and hepatoma cells. In other studies, we have found that with the differentiation of 3T3-L1 fibroblasts into cells having morphological and biochemical characteristics of adipocytes is the appearance of a particulate phosphodiesterase which exhibits a high affinity for cAMP and cGMP.