This investigation examined plasma arginine vasopressin (AVP) and aldosterone (Ald) responses to 1) oral and intravenous (IV) methods of rehydration (Rh) and 2) different IV Rh osmotic loads. We hypothesized that AVP and Ald responses would be similar between IV and oral Rh and that the greater osmolality and sodium concentration of a 0.9% IV saline treatment would stimulate a greater AVP response compared with a 0.45% IV saline treatment. On four occasions, eight men (age: 22.1 ± 0.8 yr; height: 179.6 ± 1.5 cm; weight: 73.6 ± 2.5 kg; maximum O₂ consumption: 57.9 ± 1.6 ml · kg¹ · min¹, body fat: 7.7 ± 0.9%) performed a dehydration (Dh) protocol (33°C) to establish a 4-5% reduction in body weight. After Dh, subjects underwent each of three randomly assigned Rh (back to 2% body wt) treatments (0.9 and 0.45% IV saline, 0.45% oral saline) and a no Rh treatment during the first 45 min of a 100-min rest period. Blood samples were obtained pre-Dh, immediately post-Dh, and at 15, 35, and 55 min post-Rh. Before Dh, plasma AVP and Ald were not different among treatments but were significantly elevated post-Dh. In general, at 15, 35, and 55 min post-Rh, AVP, Ald, osmolality, and plasma volume shifts did not differ between IV and oral fluid replacement. These results demonstrated that the manner in which plasma AVP and Ald responded to oral and IV Rh or to different sodium concentrations (0.9 vs. 0.45%) was not different given the degree of Dh (4.5% body wt) and Rh and amount of time after Rh (55 min).