The time courses of humoral changes in the renin-angiotensin system and the sympathetic nervous system were studied in conscious sheep in response to slow and fast hemorrhages. In two separate groups of chronically instrumented animals, 18 hemorrhages. In two separate groups of chronically instrumented animals, 18 ml/kg of blood was withdrawn over 10 or 30 minutes. The activation of the renin-angiotensin system was assessed by measurement of plasma renin activity and the sympathetic nervous system was assessed by measurement of circulating epinephrine and norepinephrine concentrations. The activation of both the renin-angiotensin system and the sympathetic nervous system occurred more rapidly in the fast hemorrhage group than the slow hemorrhage group. The peak mean plasma renin activity was 20.83 +/- 5.75 ng angiotensin I/ml/hr during the rapid hemorrhage and 8.8 +/- 1.43 ng angiotensin I/ml/hr during the slow hemorrhage (p less than 0.05). In contrast, the levels of maximal activation of the sympathetic nervous system during the slow and rapid hemorrhages were not significantly different. However, despite the threefold difference in rate of blood removal between the two groups, when the activities of the renin-angiotensin system and the sympathetic nervous system were plotted against the volume of blood removed, the time courses of change of these two humoral defense mechanisms were similar in the slow and fast hemorrhage groups. In both groups, an increase in plasma renin activity began earlier than the increase in circulating concentrations of epinephrine and norepinephrine; the maximal increase in all three humoral agents occurred near the end of the blood withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)