The increased morning frequency of cardiovascular disease onset has created concern that morning exertion promotes greater risk than evening exertion. The physiologic responses to isometric exercise (handgrip) in the morning (9 A.M.) versus the evening (6:30 P.M.) were compared in 15 subjects with mild hypertension. Isometric exercise produced similar increases at both times of the day (morning vs evening) in systolic arterial pressure (31 +/- 4 vs 35 +/- 4 mm Hg, p = NS) and heart rate (7 +/- 1 vs 8 +/- 1 beats/min, p = NS). It also produced similar increases in fibrinolytic activity (reduction in euglobulin clot lysis time) at both times of the day (-53 +/- 27 vs -53 +/- 21 minutes). However, because basal fibrinolytic activity was lower in the morning than in the evening (euglobulin clot lysis time 364 +/- 45 vs 220 +/- 33 minutes, p < 0.01), peak fibrinolytic activity after stress was also lower in the morning (312 +/- 44 vs 176 +/- 27 minutes, p < 0.01). The present study demonstrates that in subjects with hypertension, the hemostatic and hemodynamic responses to handgrip are not greater during morning versus evening exertion. This finding, plus the overall benefits of regular exercise and the low absolute risk of an event during exercise, suggests that timing of exertion is not of critical importance. However, to obtain a definitive answer to this question, further studies are needed using different potential triggers and subject populations.