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Cyclic guanosine monophosphate (cGMP)-phosphodiesterase (PDE) in the outer segment of vertebrate retinal rod cells is one of key enzymes mediating phototransduction. We report here on light-induced PDE activity in intact rat retinal rod cells processed by rapid-freezing enzyme cytochemistry, a new morphological technique that is a combination of rapid-freezing, freeze-substitution fixation, and subsequent enzyme cytochemistry. This technique quickly immobilizes and preserves both enzyme activity and the cell ultrastructure in a state approximating living conditions; consequently, it has proved useful for cytochemical detection of light-induced PDE activity. Using this technique we observed that the catalytic site of PDE molecules in rapidfrozen outer segments was predominantly located in the extradiscal spaces; PDE activity was significantly greater in light than in darkness; and illumination elicited marked increases in PDE activity in dark-adapted cells. Light-induced PDE activity was first cytochemically detected after 3 s of illumination and reached a peak within 5 s, at which time it was in virtually the same as that seen in fully light-adapted cells. In addition, cytochemical guanosine triphosphatase (GTPase) activity in dark-adapted cells, as well as corresponding PDE activity, increased in a time-dependent manner with illumination duration; this acceleration in GTPase activity closely paralleled the PDE activity. Thus, our results suggest, in part, the existence of reciprocal regulation of PDE and activated transducin α subunit, thereby modulating light adaptation in rod cells. [ABSTRACT FROM AUTHOR]