The adenosine 3', 5'-cyclic monophosphate (cyclic AMP)-dependent protein phosphokinase of rat interstitial cells was characterized by ion-exchange chromatography and sucrose density gradient centrifugation. The 0.2 M NaCl fraction from DEAE-Sephadex showed a small 2.9-S peak of basal enzyme activity, and a large 6.5-S peak of cyclic AMP-dependent protein kinase activity; fractions eluted from DEAE-Sephadex with 0.3-0.5 M NaCl contained a major 3.8-S peak of cyclic AMP-dependent enzyme activity. Activation of protein kinase in cell extracts by cyclic AMP, and in intact interstitial cells by trophic hormone, caused a major shift of enzyme activity to the 2.9-S cyclic AMP-dependent form which was eluted from DEAE-Sephadex by 0.2 M NaCl. These results are consistent with the presence of two distinct protein kinase holoenzymes, with a common 2.9-S catalytic subunit. During hormonal activation of protein kinase in dispersed interstitial cells by 10-10 M human chorionic gonadotropin (hCG), conversion to the 2.9-S catalytic subunit was observed between 2 and 30 min of incubation. Protein kinase activity was correlated with cyclic AMP production, and full enzyme activation occurred at the time of maximum intracellular cyclic AMP concentration. The presence of two forms of cyclic AMP-dependent protein kinase in the Leydig cell provides a potential mechanism whereby progressive occupancy of gonadotropin receptors could evoke a series of discrete target cell responses.