Localization of 3', 5'-cyclic nucleotide phosphodiesterase activity in thyroid follicular cells and endothelium was examined by cytochemical methods for electron microscopy. In thyroid follicular cells reaction product deposition associated with 3', 5'-cyclic nucleotide phosphodiesterase activity was localized at the plasma membrane. In these cells, the enzyme activity appeared mostly localized on the apical and lateral plasma membrane, and also appeared on the outer surface of nuclear envelope, endoplasmic reticulum and Golgi apparatus. However, when cAMP and snake venom were omitted simultaneously, no enzyme activity was observed. 3', 5'-cyclic nucleotide phosphodiesterase activity also appeared on the luminal surface of endothelium of small blood vessels.