Type B photoreceptors in the eyes of the nudibranch mollusc Hermissenda have previously been shown to exhibit enhanced steady-state depolarizing light responses, increases in steady-state light-induced impulse frequency and a decrease in resting membrane conductance on days following exposure to repeated pairings of light and rotation. These training-produced changes in the electrophysiological properties of B cells have been attributed to reductions in a voltage-dependent K+ current (IA), a calcium-activated K+ current (IK-Ca), and enhancement of a voltage-dependent Ca2+ current (ICa). Current- and voltage-clamp analysis of the effects of serotonin (5-HT) upon B cells revealed that 5-HT mimicked many of the effects of associative training. 5-HT enhanced the steady-state light response and decreased resting membrane conductance of B cells. Voltage-clamp analysis revealed five distinct effects of 5-HT upon the voltage-, calcium-, and light-dependent ionic conductances of B photoreceptors. In the dark-adapted cell, 5-HT enhanced but then suppressed IA, IK-Ca, and enhanced ICa. The presentation of 5-HT in combination with light accelerated the reduction of K+ currents. 5-HT also transiently reduced the fast component of light-induced inward current (INa-light), and enhanced a slower component of light-induced inward current. In an accompanying paper, 5-HT is shown to be localized within the cerebropleural neuropil where terminals presynaptic to B photoreceptors are located. Disruption of serotonergic neurotransmission by a variety of drugs is also shown to reduce the pairing-specific differences in Type B photoreceptor cumulative depolarization and resting membrane conductance decreases that are produced by in vitro conditioning. Collectively, the results indicate that 5-HT plays an important role in mediating the conductance changes produced in Type B photoreceptors by associative training.