The present study was designed to test the hypothesis that gender and female reproductive cycle phase influence the restitution of blood volume following blood loss. The experiments were performed in conscious 9- to 11-wk-old Sprague-Dawley rats subjected to a slow hemorrhage of 19 ml/kg over 40 min. The effect of hemorrhage was compared in male rats and in female rats in either proestrus (Pro) or metestrus (Met). In comparison with either metestrus females or males, females hemorrhaged on the morning of proestrus showed a significantly larger overall decrease in percent hematocrit (Pro, -17 +/- 1; Met, -10 +/- 1; male, -13 +/- 1; mean +/- SE), increase in plasma volume (Pro, to 151 +/- 15% of initial volume; Met, 104 +/- 8%; male, 120 +/- 4%), and increase in total plasma protein content (Pro, to 164 +/- 14% of initial content; Met, 101 +/- 5%; male, 132 +/- 5%) over the 21-h posthemorrhage recovery period. Proestrus females also showed a larger increase in plasma osmolality during the first 2.5-h posthemorrhage than either metestrus females or males (P < 0.05). In addition, basal hematocrit and total blood volume (51Cr-tagged erythrocyte method) were higher in females studied on the morning of proestrus than in metestrus females. These results indicate that the female reproductive cycle is an important variable in basal blood volume regulation and in plasma protein and plasma volume restitution following blood loss.