Female rats drink more 3% NaCl solution than do males, both when they need sodium (need-induced sodium intake or sodium appetite) and when they do not (need-free sodium intake). The sexual dimorphism of sodium intake is a secondary sexual characteristic because after castration at 1 day of age male rats drank 3% NaCl in adulthood in a manner similar to that of females in both the need-free and need-induced state, and females given long-term, neonatal testosterone drank low, malelike volumes of 3% NaCl on a daily need-free basis, but their response to sodium depletion was unchanged. This sexual dimorphism of sodium intake seems to be governed by testosterone that has been converted in the brain to estrogen because treatment of Day 1 castrated females with a nonaromatizable androgen, dihydrotestosterone, did not change either their need-free or their need-induced 3% NaCl intake. Castration in adulthood of male and female rats did not change their sodium consumption. However, when castrated adults received testosterone, need-free intakes of NaCl were suppressed in both sexes, but the suppression of 3% NaCl intake occurred only while the steroid was present. Exogenous testosterone also lowered the need-induced sodium intake of adult castrated females. Thus, in castrated adults, need-free intake was actively suppressed by exogenous testosterone in both sexes, whereas need-induced intake of NaCl was suppressed only in females. These data indicate that sodium intake in the rat is a sexually dimorphic behavior that is organized neonatally and can be actively suppressed in adulthood by testosterone.