Recent studies suggested a direct link between circadian rhythms and regulation of sodium excretion. Endothelin-1 (ET-1) regulates sodium balance by promoting natriuresis through the endothelin B receptor (ETB) in response to increased salt in the diet, but the effect that the time of day has on this natriuretic response is not known. Therefore, this study was designed to test the hypothesis that ETB receptor activation contributes to the diurnal control of sodium excretion, and that sex differences contribute to this control as well. 12-hour urine collections were used to measure sodium excretion. On day 3 of the experiment, a NaCl load (900 µEq) was given by oral gavage either at Zeitgeber [ZT] time 0 (inactive period) or ZT12 (active period) to examine the natriuretic response to the acute salt load. Male and female ETB deficient (ETB def) rats showed an impaired natriuretic response to a salt load at ZT0 compared to their respective transgenic controls (Tg cont). Male ETB def rats showed a delayed natriuretic response to a salt load given at ZT12 compared to male Tg cont, a contrast to the prompt response shown by female ETB def rats. Treatment with ABT-627, an ETA receptor antagonist, improved the natriuretic response seen within the first 12 hours of a ZT0 salt load in both sexes. These findings demonstrate that diurnal excretion of an acute salt load 1) requires ET-1 and the ETB receptor, 2) is more evident in male versus female rats, and 3) is opposed by the ETA receptor.