1. The Ca(2+)-dependent K+ current (IK,Ca) contributes to both the plateau phase of light-elicited generator potentials and enhanced excitability of identified type B photoreceptors of Hermissenda detected after classical conditioning. Serotonergic modulation of membrane conductances mimics some of the effects of conditioning. Serotonin (5-HT) reduces the magnitude of IK,Ca and decreases the sustained voltage-activated Ca2+ current (ICa) in type B photoreceptors. We have examined the modulatory role of 5-HT in regulation of IK,Ca by ICa using a Ca2+ ionophore in conjunction with the whole cell patch-clamp technique in isolated photoreceptors. 2. The 40-50% reduction of IK,Ca by 5-HT was voltage independent. Cd2+ blocked ICa and reduced IK,Ca by 70-80%. The remaining 20-30% of IK,Ca may result from Ca2+ release from intracellular stores, because IK,Ca was further reduced to 5-10% as the pipette ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) was raised from 0.5 to 5 mM. The application of the Ca2+ ionophore A23187, which was designed to produce Ca2+ influx independent of the voltage-activated Ca2+ channels, restored IK,Ca. 3. The application of A23187 reversed the effects of 5-HT and Cd2+ on IK,Ca for experiments lasting 15-20 min. However, for longer time periods (> 25 min), complete restoration of IK,Ca by A23187 was obtained in the presence of Cd2+ but not 5-HT. These results suggest that for 15 to 20 min exposures the reduction of IK,Ca by 5-HT is a consequence of modulation of ICa by 5-HT and not a direct effect of 5-HT on IK,Ca.