Low-dose infusions of dexamethasone (2 micrograms/day for 2 and 4 weeks) increased systolic blood pressure and decreased body weight gain in male rats. Total body sodium, calcium and magnesium were increased by dexamethasone treatment; potassium was unaffected. These changes have been evaluated bearing in mind that glucocorticoids have profound catabolic effects. Relative to pretreatment values, dexamethasone decreased exchangeable body sodium for the first two weeks of treatment although values were not significantly different from vehicle-treated controls. Hematocrit, plasma cholesterol and transaminase activities were increased by dexamethasone; white cell numbers and plasma volumes were decreased; plasma Na+, K+ and Ca2+, red cell numbers, extracellular fluid volume, and glomerular filtration rate were not significantly affected. It is concluded that glucocorticoids cause plasma volume to contract, possibly as a result of glucocorticoid-induced natriuresis. Any effects of dexamethasone on whole-body ionic composition are obscured by simultaneous changes of intermediary metabolism.