Twelve women (23-34 yr), comprising a bed-rest (BR) group of 8 subjects and an ambulatory (AMB) group of 4 subjects, were centrifuged after 14 days of ambulatory control (C), after 15 days of a 17-day BR period, and on the 3rd day of recovery (R). Venous blood was taken before and after the 3rd +3.0 G acceleration run (1.8 G/min). Relative to (C), the +Gz tolerance after BR was reduced -49.0% (P < 0.05) in the BR group and -38.7% (NS) in the AMB group; during (R) the BR group regained up to 89.4% and the AMB group up to 87.1% of their (C) tolerances. In each of the 3 test periods, the shifts in plasma Na, Cl, PO4 and osmotic contents, which accompanied +Gz, followed the outward shift of plasma volume (PV). The correlation of the shift of PV during acceleration with the +Gz tolerance was 0.72 (P < 0.01). During acceleration, the PV and electrolyte loss for both groups after BR was about half the loss of (C) and (R). Compared with (C) and (R) values, K shifts were variable, but the mean corpuscular volume and mean corpuscular HB contents and concentrations were unchanged during all +Gz runs. The higher the (C) +Gz tolerance, the greater the tolerance decline due to BR; relative confinement and reduced activity contribute as much to the reduction in tolerance as does the horizontal body position during BR; bed-rest deconditioning has no effect on the erythrocyte volume during +3.0Gz; and about 1/2 the loss in tolerance after BR can be attributed to PV and electrolyte shifts.