The aim of this work was to study the influence of beta-adrenoreceptor blockade on the adaptation to exercise of one of the hormonal systems (arginine vasopressin) involved in the regulation of blood volume and pressure in spontaneously hypertensive rats (SHR). Systolic blood pressure (SBP) was measured in SHR and WKY rats during 11 wk of swim training. At the end of the training program we determined post-exercise values of plasma arginine-vasopressin (pAVP), osmolality (pOsm), K+ (pK+), Na+ (pNa+), hemoglobin (Hgb), and hematocrit (Hct) in SHR and WKY rats. The following groups were studied: control (C), propranolol treated (PC), swim trained (S), and propranolol-treated and swim-treated (PS). SBP was significantly reduced by swim training or propranolol, bu these beneficial effects on SBP were attenuated when propranolol and swim training were combined. pNa+ and pOsm were significantly reduced by training alone in SHR. This reduction of pNa+ and, consequently, of pOsmol without any modification of other parameters could suggest an Na+ loss. In contrast, the SHR group treated with propranolol alone showed a significant reduction in Hct, suggesting an increased plasma volume without Na+ loss. PS SHR showed a significant reduction of Hgb, Hct, proteins, pNa+, and pOsmol, probably as a consequence of the additive effects of swimming- and propranolol-induced hypervolemia with Na+ loss. The slight and nonsignificant reduction in pAVP observed with either training or propranolol treatment alone became much more pronounced and statistically significant when the 2 treatments were combined. WKY rats showed a much smaller response to exercise and beta-adrenoreceptor blockade than SHR. We conclude that the hypervolemia suggested in PS SHR could be a possible cause of attenuation of the beneficial effects of either swimming or propranolol on SBP.