This study examines whether changes in cGMP concentration initiated by illumination of frog rod photoreceptors occur rapidly enough to implicate cGMP as an intermediate between rhodopsin activation in the disc membrane and permeability changes in the plasma membrane. Previous studies using whole retinas or isolated outer segments have provided conflicting evidence on the role of cGMP in the initial events of phototransduction. The rod photoreceptor preparation employed in this work consists of purified suspensions of outer segments still attached to the mitochondria-rich ellipsoid portion of the inner segment. These photoreceptors are known to retain normal electrophysiological responses to illumination and have cGMP levels comparable to those measured in the intact retina. When examined under several different conditions, changes in cGMP concentrations were found to occur as rapidly or more rapidly than the suppression of the membrane dark current. Subsecond changes in cGMP concentration were analyzed with a rapid quench apparatus and confirmed by comparison with a rapid freezing technique. In a 1 mM Ca2+ Ringer's solution, cGMP levels decrease to 65% of their final extent within 200 ms after bright illumination; changes in membrane dark current follow a similar time course. When the light intensity is decreased to 8000 rhodopsins bleached per rod per s, the light-induced cGMP decrease is completed within 50 ms, with 7 X 10(5) cGMP molecules hydrolyzed per rhodopsin bleached. During this time the dark current has not yet begun to change. Thus, under physiological conditions it is clear that changes in cGMP concentration precede permeability changes at the plasma membrane. The correlation of rapid changes in cGMP levels with changes in membrane current leave open the possibility that changes in cGMP concentration may be an obligatory step in the reaction sequence linking rhodopsin activation by light and the resultant decrease in sodium permeability of the plasma membrane.