The postnatal developmental patterns of sigma (sigma) and phencyclidine (PCP) binding sites were compared in the rat brain. The results show diametrically different ontogenic patterns for the sites. While both the affinity and the density of sigma sites remain constant throughout the developmental period tested (postnatal day 1 to 1 year), the density of PCP binding sites increases from the time of birth, reaching the adult level by postnatal day 14. The differences in developmental patterns provide evidence for distinctive properties of cerebral sigma and PCP binding sites.