Calcium currents and the effects of 5-HT on these currents were investigated in visually identified motoneurons using whole-cell recording in the neonatal rat spinal cord slice preparation. In current- clamp recording, step depolarizations from a holding potential of about -90 mV produced a low-threshold transient depolarizing response and a high-threshold long-lasting spike. In voltage-clamp recording, low (LVA) and high (HVA) voltage-activated Ca2+ currents were recorded in response to depolarizing voltage steps. Low concentration of Cd2+ (50 microM) did not reduce the amplitude of the LVA current but markedly diminished the HVA current. Bath application of 5-HT (10–50 microM) markedly increased the amplitude of the LVA current without causing a shift in the current (I)-voltage (V) relation. In contrast, 5-HT did not appreciably affect the amplitude of the HVA current. We conclude that 5-HT specifically enhances the LVA Ca2+ current and that this effect together with the previously reported 5-HT-induced inward current (Takahashi and Berger, 1990), would facilitate the excitation of motoneurons.