Developmental effects of a single hydrocortisone (HC) injection (80 micrograms/g on day 2 postnatal) were compared in two closely related species, meadow voles (Microtus pennsylvanicus) and pine voles (M. pinetorum). Effects of hormone administration were assessed in terms of the development of swimming behavior (head/nose position and front paw movement), alterations in social interactions (spatial location with respect to a stimulus animal), and by changes in somatic and brain growth. Although adult patterns of swimming behavior were found to be quantitatively different in meadow and pine voles, normal maturation in both species involved a progressive elevation of head/nose position along with a gradual inhibition of front paw movement. Compared with saline controls, HC-treated meadow voles showed an accelerated attainment of their adult pattern, whereas hormone-treated pine voles displayed a retardation in their swimming development. HC administration similarly led to increased body weights in developing meadow voles but decreased body weights in young pine voles. There was a tendency for hormone-treated meadow voles to spend more time near a stimulus animal than their control counterparts, whereas the opposite was observed for pine voles. In both species, the HC group displayed decreased cerebral and cerebellar weights in adulthood, whereas other organs were generally unaffected by hormone administration. Neurochemical analyses revealed no treatment effects or treatment by species interactions with respect to cerebral DNA levels. On the other hand, treated meadow voles but not pine voles showed an elevated concentration of sulfatide, a myelin-related lipid. These results are the first to demonstrate divergent behavioral and neurochemical effects of early glucocorticoid administration in closely related species studied under the same conditions. Furthermore, in accordance with earlier experiments on rats, swimming behavior was shown to be a useful measure for assessing changes in neuromuscular maturation.