To study the effect of prolonged recumbency on plasma vasopressin and renin activity, 8 women (23-34 yr) were subjected to 17 days of absolute bed rest. The +3 Gz tolerance of the subjects was tested before and after 14 days of bed rest. From day 2 and through day 17 of bed rest, plasma arginine vasopressin (AVP) levels were reduced 33%. Plasma renin activity (PRA) increased 91% (P < 0.05) above ambulatory control values from days 10 through 15 of bed rest. When compared to precentrifuge values, exposure to +3 Gz perior to bed rest provoked a 20-fold rise (P < 0.05) in mean plasma AVP but resulted in only a slight increase in PRA. After bed rest acceleration increased plasma AVP 7-fold (P < 0.02); the magnitude of this increase was less than the post +3 Gz value obtained prior to bed rest. After bed rest no significant rise was noted in PRA following +3 Gz. Prolonged bed rest leads to a significant rise in the PRA of female subjects, while exposure to +Gz acceleration provokes a marked rise in plasma AVP.