Female Sprague-Dawley-derived rats were injected at 25 days of age (pre-pubertal) or 60 days of age (adult) with 1 µg dexamethasone/100 g b.w. either before the beginning of the daily rise in serum corticosterone (at 10.00 or l2.OOh in adults and 10.00 h in prepubertal rats) or after the daily rise had begun (at 14.00 h in adults, and 12.00 or 14.00 h in prepubertal rats). Blood samples were collected by decapitation at 16.00, 20.00 and 24.00 h on that day and 04.00, 08.00 and 16.00 h the following day. In adults, dexamethasone (DEX) given at 10.00 shifted the corticosterone (B) peak to 20.00 from the control peak at 16.00 h. DEX at 12.00 or 14.00 h shifted the peak to 04.00. In prepubertal rats, DEX given before the B rise did not shift the subsequent peak and the pattern of B did not diverge from controls. DEX given at 12.00 or 14.00 shifted the peak to 24.00 h. At 08.00 the next day, B was depressed in adults but normal in prepubertal rats. At 16.00 h, both age groups showed depressed B in comparison to controls. Prepubertal rats appear to respond differently to dexamethasone than do adults.