The effects of alterations of plasma osmolality on plasma arginine vasopressin (AVP) and renin activity (PRA) following graded hemorrhage were studied in conscious dogs who were either euhydrated, dehydrated, water loaded, or infused with hypertonic saline. Base-line plasma osmolality and AVP were significantly different in the four treatment groups; however, following hemorrhage the increases in log AVP did not significantly differ. An unexpected finding was that water loading resulted in significant elevations in PRA and plasma aldosterone concentrations, whereas plasma osmolality and AVP were reduced. Prior to hemorrhage, PRA was significantly greater in the water-loaded and dehydrated groups than in the euhydrated or saline-infused groups; following hemorrhage the increases in log PRA were not significantly different in all four treatment groups. The data suggest that, although alterations in osmolality influence base-line levels of AVP, they have no effect on relative (logarithmic) rises in AVP following hemorrhage. Similarly, alterations in AVP may influence base-line PRA, but do not influence relative rises in PRA following hemorrhage.