An in vitro slice preparation was used to obtain intracellular recordings of neostriatal neurons. Indirect evidence for the presence of an early outward conductance in neostriatal neurons is presented. With near threshold stimulation neostriatal neurons fired very late during the pulse. The long firing latency was associated with a slow (ramp-like) depolarization. In the presence of TTX the slow depolarization was lost and outward-going rectification dominated the subthreshold response. This finding demonstrated that both, outward- and inward-going conductances play a role during the ramp-like depolarization. Outward-going rectification during depolarizing responses could be further augmented if the depolarizing stimulus was preceded by a conditioning hyperpolarization. A conditioning hyperpolarization prolonged the firing latency and slowed the firing frequency. A conditioning depolarization had opposite effects. After TTX treatment, the response showed a hyperpolarizing "sag" when depolarizing stimulation was preceded by conditioning hyperpolarization. 4-AP (0.5-2.5 mM) blocked the effects of the conditioning hyperpolarization on the firing latency and on the voltage trajectory. 4-AP also disclosed a slow depolarization which could produce neuronal firing very early during the pulse. This depolarization was TTX-sensitive and Co++-insensitive. In contrast to 4-AP, TEA (20 mM) did not produce a reduction in the firing latency but disclosed a membrane oscillatory behavior most probably produced by the interplay of these opposing conductances: the slow inward (probably Na+) and the transient outward (probably K+). Repetitive firing during 4-AP treatment was of the "phasic-tonic" type with an initial burst riding on the initial Co++-insensitive slow depolarization and a somehow irregular train of spikes during the remainder of the stimulation. Action potentials during 4-AP treatment were followed by an afterdepolarization which dominated the initial part of the interspike interval.