The influence of maternal corticosterone during lactation on the development of the hippocampal corticosteroid receptor system, hypothalamus-pituitary-adrenal axis activity and spatial learning/retention performance was investigated in the rat during postnatal days 11 to 30. We increased the plasma levels of corticosterone by adding the hormone (200 microg/ml) to the drinking water of the dams. When compared to controls corticosterone-nursed offspring displayed: i) higher number of hippocampal type I and type II corticosteroid receptors at 30 days of life, but no changes at 11 and 16 days; ii) higher plasma levels of corticosterone in the basal condition and after 15 min of maternal separation at 11 but not at 16 days: iii) lower adrenal weights at 11 and 16 days, but which were no longer present at the age of 30 days; iv) no difference in performance in the place learning version of the Morris water task and T aquatic maze at 16 days. The present results, together with our previous findings showing that 90-day-old corticosterone-nursed rats have lower basal and restraint stress corticosterone levels and improved learning performance, indicate that the effects of maternal treatment appears only after weaning, thereby suggesting that increased corticosteroid receptors may be responsible, at least partially, for the endocrine and learning modifications induced by pre-weaning corticosterone exposure. The role played by maternal circulating corticosterone during the period of lactation in shaping the characteristics of the hypothalamus-pituitary-adrenal axis and brain of the offspring is outlined.