Intracellular electrophysiological recordings in current- and voltage-clamp mode were obtained from dopaminergic neurons of the rat mesencephalon in an in vitro slice preparation. In current-clamp mode, a time-dependent anomalous rectification (TDR) of the membrane was observed in response to hyperpolarizing current pulses. In single-electrode voltage-clamp mode, a slowly developing inward current (lh) underlying the TDR was studied by hyperpolarizing voltage commands from a holding potential of -50 to -60 mV. lh started to be activated at -69 mV, was fully activated at -129 to -141 mV, with half-maximal activation at -87 mV, and showed no inactivation with time. The time course of development of Ih followed a single exponential, and its time constant was voltage-dependent. At -81 mV, lh activated with a time constant of 379 2 47.6 ms, whereas at -129 mV lh activated with a time constant of 65 ? 2.2 ms. Its estimated reversal potential was -35 ± 4 mV. Raising the extracellular concentration of K+ from 2.5 to 6.5 and to 12.5 mM increased the amplitude of lh while reducing the extracellular concentration of Na+ from 153.2 to 27.2 mM caused a reduction in amplitude of lh. Bath application of caesium (1–5 mM) reversibly reduced or blocked the TDR/lh. Perfusion of tetrodotoxin (0.5–1 μM), tetraethylammonium (10–20 mM) or barium (0.3–2 mM) did not significantly affect lh. lh was also present in cells impaled with CsCI-filled electrodes. When lh was substantially reduced by extracellular caesium (1 mM) the firing rate of the dopaminergic cells, which consisted of a spontaneous pacemaker discharge of action potentials, was not clearly changed. In addition, the holding current in voltage-clamp experiments at -50 to -60 mV was not affected by 1 mM caesium. We conclude that although the lh current is a typical feature of the dopaminergic neurons, it is neither a significant factor underlying the spontaneous pacemaker activity nor does it contribute substantially to the setting of the normal resting potential level of the membrane. On the other hand, since lh starts at voltages lower than or equal to -69 mV (below firing threshold), it may play a modulatory role in the cell's excitability by limiting the amplitude and duration of any prolonged hyperpolarizing events in the dopaminergic cells.