Forskolin, a direct activator of adenylate cyclase, modifies the voltage-dependent K+ conductance of quiescent human peripheral blood T lymphocytes. In the presence of greater than 20 microM forskolin, the average voltage-gated current in whole-cell patch clamp is significantly decreased. The voltage dependence and kinetics of activation are not changed from untreated control cells. However, inactivation becomes biphasic. Much of the current inactivates very quickly (complete in 10 ms), and the remaining outward current inactivates more slowly with a time constant closer to that of control cells. To determine whether this effect is mediated by a rise in intracellular cAMP, cells were preincubated and subsequently voltage-clamped in the presence of other agents that raise the cAMP levels in T cells (isoproterenol plus a phosphodiesterase inhibitor, or dibutyryl cAMP) with no effect on the K+ conductance. Similarly, cells put in whole-cell patch clamp with cAMP, GTP, ATP, and theophylline added to the electrode filling solution showed no change in K+ current. Because other procedures that raise cAMP did not duplicate the effect of forskolin, we investigated the effect of 1,9-dideoxyforskolin, an analogue of forskolin that does not stimulate adenylate cyclase in human lymphocytes. This drug induced changes in the whole-cell K+ conductance identical to those observed with forskolin. Both forskolin and dideoxyforskolin inhibit mitogen-induced proliferation of lymphocytes. Because inhibition of proliferation occurs in the presence of known K+ channel blockers, these results suggest that forskolin has an effect on T cell mitogenesis that is mediated by inhibition of K+ conductance and is independent of cAMP.