The localization of cyclic 3',5'-phosphodiesterase in the superior cervical ganglion of the rat was studied by an ultrastructural cytochemical method. Large amounts of evenly distributed electron dense granular reaction products were detectable on the cytoplasmic processes of Schwann cells while a discontinuous and rough reaction product was observed in the postganglionic neurones. Usually, a significant number of lead phosphate deposits was found in the capillaries but endothelial cells sometimes were free from the reaction product. Abundant lead phosphate deposits were accumulated in the long processes of macrophages situated in the connective tissue of the ganglion. In the synaptic and non-synaptic neuropil, fine granulated reaction products were associated with microtubular and microfilamentar elements, pre- and postsynaptic membrane thickenings, mitochondria, rough surface endoplasmic reticulum, and multivesicular bodies as well. The wide ranging occurrence of cyclic phosphodiesterase positive reaction product in the superior cervical ganglion reflected the fundamental intra and extracellular regulatory role of this enzyme in cyclic nucleotide metabolism.