Quantitative morphological characteristics of cells in the primary somatosensory cortex of 6-day-old rats were examined following continuous maternal administration of morphine (10 mg/kg/day), naltrexone (10 mg/kg/day), or saline vehicle from gestation day 12. Naltrexone reduced neuronal packing density and significantly increased cortical thickness but had not effect on neuronal number, while morphine reduced neuronal packing density and the number of neurons without affecting cortical thickness. These results suggest that blockade of endogenous opioid function during development enhances neuronal maturation in this brain region, while perinatal morphine administration might act to restrict cortical cell proliferation and maturation. Thus, the effect of ontogenetic activation of opioid receptors by endogenous opioid compounds could be similar to but less severe than the effect of exogenous opiate exposure on cortical cell development.