Development of striatal unit activity recorded from chloral hydrate anesthetized, neonatal rats was characterized electrophysiologically following acute or repeated haloperidol administration. No spontaneously active single units were detected in 8 day old pups. Spontaneous activity was recorded by 17 days of age, although the number of active cells, firing frequency and the variety of firing patterns were less diverse than those observed in 28 day olds. There were also age related differences in striatal unit responses to haloperidol. A significant increase in activity was induced by acute haloperidol administration only in 28 day old animals. No tolerance to the acute effects was demonstrated. Both 17 and 28 day olds responded to repeated haloperidol injections, followed by a 24 h recess, with an increase in striatal activity. These results may assist our understanding of the effects of human fetal, neonatal and/or adolescent exposure to neuroleptics.