The morphological effects of two chemically different neuroactive drugs (chlorpromazine and phenobarbital) on developing Purkinje cells in the rat cerebellum were examined to determine the presence of cytological alterations. Therapeutic dosages of both durgs were chronically administered to separate groups of maternal rats beginning on day 18 postcoitus. Entire litters were sacrificed on postnatal days 13, 15, 18, and 21. Light microscopic quantitation of Purkinje cells demonstrated a statistically significant reduction in total numbers below control levels at all ages examined for both drugs. Pyknotic Purkinje cells, which appeared more numerous in the drug tested groups, had a paucity of synaptic contacts on both the soma and dendritic branches. Both drugs tested produced similar results within the parameters examined. These alterations have been discussed in relation to possible mechanisms and sites of action.