Pregnant rats received daily injections of 6 or 40 mg/kg progesterone or vehicle only from days 1 to 16 of gestation. The morphology of pyramidal cells in the parietal cortex of the offspring on days 5 and 25 was assessed after staining by the Golgi technique. At postnatal day 5 the cells from both progesterone groups showed a 16% increase in branching of basal dendrites but no change in dendritic spread or cell diameter. At day 25 the number of spines in the apical dendrites of layer IV and V cells were increased 24% in the low-dose group and 36% in the high-dose group. Moreover, the high-dose group showed a significant increase in brain weight on day 25. The amplitude and latency of the cortical evoked potentials on day 5 were not altered by the progesterone treatment. We conclude that progesteone can influence cortical cell morphology but there was no evidence for modification of developing somatosensory pathways.