Spontaneous firing patterns of 94 unidentified neurons and 34 identified spiny neurons were compared in the neostriatum of locally anesthetized immobilized rats. Intracellular and extracellular recordings were analyzed using first order interval histograms and autocorrelograms, and neurons were identified by their somatodendritic morphology after intracellular injection of horseradish peroxidase. All neostriatal neurons tended to fire in irregular phasic bursts of activity. Considerable variation in mean firing rate, burst duration, interburst interval and the occurrence and rate of firing between bursts was apparent in both groups of neurons. There was no apparent difference between spiny neurons and the sample of unidentified extracellularly recorded neurons along any of these firing pattern parameters. Intracellular recordings from identified spiny neurons revealed noisy irregular periods of maintained 5-20 mV membrane depolarizations which corresponded to the occurrence of bursts of firing in spontaneously active neurons. These depolarizations occurred in neostriatal neurons exhibiting no spontaneous activity but were of insufficient amplitude to trigger impulse activity.