1. Intracellular recordings from neostriatal neurons in an in vitro slice preparation of the rat brain were used to analyze the pharmacological sensitivity of the action potential (AP) repolarization and the afterhyperpolarization (AHP) that follows a single action potential. The interspike voltage trajectory and the AHP could be divided into two main parts: a fast component lasting a few milliseconds and better observed during a train of spikes, and a slow component lasting approximately 250 ms and that comprises the main portion of the AHP. In some cells, a slow (up to 1 s) component of low amplitude was also detected.2. Single APs were elicited at two imposed membrane potentials (around -60 and around -80 mV). The AP amplitude was larger, the repolarization rate was faster, and the duration was shorter when spikes were evoked at -80 mV. When measured from the -60 mV holding potential, the afterpotential was an AHP with peak amplitude of -5 mV. The afterpotential became a delayed depolarization (DD) at -80 mV.3. Firing frequency adaptation was voltage sensitive. The firing of APs induced by long intracellular current pulses from a holding potential of -80 mV exhibited only a slow-frequency adaptation (time constant of seconds). However, at -60 mV, an initial and faster frequency adaptation was evident (time constant of tens of milliseconds).4. The Ca2+ channel blocker Cd2+ retarded AP repolarization rate. This effect correlated with a significant block of the fast and slow components of the AHP. In contrast, Ni2+ had no significant effects on the same parameters. Cd2+, but not Ni2+, induced an increase in the firing frequency for a given stimulus. Frequency versus time plots still exhibited spike frequency adaptation after Cd2+ effects.5. Both apamin and charybdotoxin (CTX) reduced the peak amplitude of the AHP after a single AP. Apamin increased the firing frequency for a given stimulus. Apamin also reduced the slow AHP that follows a train of spikes, whereas CTX had much less effect, if any, after a train of spikes. CTX was effective in retarding AP repolarization, whereas apamin did not affect it.6. Tetraethylammonium (TEA) also delayed AP repolarization and reduced the AHP after a single spike at 1 mM. Both CTX and TEA did not significantly affect firing frequency, adaptation, or the posttrain AHP when the stimulus was a rectangular current pulse of moderate intensity. During repetitive firing, the first interspike voltage trajectories were the most affected by both substances.7. Because some frequency adaptation remains after the blockade of Ca2+ entry by Cd2+ or apamin, we conclude that conductances that are not Ca2+ sensitive may also be partly responsible for frequency adaptation in these neurons.