A two-microelectrode potential clamping method was used on isolated common snail neurons to measure high-threshold Ca2+ and delayed rectified K+ currents. Addition of the nootropic agent vinpocetine (VPC) to the bathing solution rapidly and reversibly inhibited both types of current. The effects of VPC were dosedependent and were independent of the test stimulus voltage. Maximum blockade of the Ca2+ current averaged 27% at a VPC concentration of 600 μM. Maximum blockade of the K+ current averaged 76% at a VPC concentration of 30 μM. It is concluded that K+ channels are more likely targets of VPC than Ca2+ channels.