In the forebrain of 56-day-old rats, histochemical studies revealed that the subcutaneous injection of a psychotomimetic phencyclidine (PCP; 1 and 10 mg/kg) induced a dose-related and dense nuclear c-Fos-like immunoreactivity in the pyriform cortex, layers IV-VI of the neocortex and septum, but a sparse c-Fos immunostaining in the olfactory tubercle and mid-lateral striatum. Infant rats at postnatal day 8 expressed much fewer and more confined c-Fos-positive cells in the neocortex than young adult rats following PCP injection. However, a similar expression pattern of PCP-induced c-Fos was observed in the pyriform cortex, mid-lateral striatum, olfactory tubercle and septum between the infant and adult periods. These developmental changes in the regional distribution of a neuronal activity marker, c-Fos, suggest that neuronal populations involved in PCP-induced abnormal behavior are influenced by postnatal development, at least, in the neocortex.