The mechanisms underlying the higher incidence of cardiovascular events in the morning were investigated by studying the circadian variation in hemodynamics and baroreflex sensitivity (BRS) in 20 untreated inpatients with essential hypertension. Direct blood pressure (BP) and heart rate (HR) were recorded using telemetry. Cardiac output (CO) was measured by the dye dilution method. Beat-to-beat stroke volume (SV) and total peripheral vascular resistance (TPR) were obtained using the pulse contour method. The coefficient of regression between HR and systolic BP (SBP) change (delta HR/delta SBP = Ahr) was calculated for in 5 consecutive heart beats during which BP decreased spontaneously and linearly (r > 0.9). Similarly, the delta SV/delta SBP (= Asv) and delta TPR/delta SBP (= Atpr) were also measured, and the negative values of these coefficients (-Ahr, -Asv and -Atpr) were calculated. Comparisons between morning (6-11 a.m.) and evening (4-9 p.m.) values showed no significant difference in mean BP (122 vs 127 mmHg) and HR (72 vs 73 bpm). However, CO (3.7 vs 4.2 l/min), -Ahr (0.28 vs 0.43 bpm/mmHg) and -Asv (-1.5 vs 1.4 ml/mmHg) were lower in the morning than in the evening (p < 0.01). In contrast, TPR (40 vs 34 mmHg/l/min) and -Atpr (1.2 vs -1.4 min/l) were higher in the morning than in the evening (p < 0.01). These findings suggest that lower -Ahr and -Asv and higher TPR and -Atpr may cause stress to the cardiovascular system in the morning in patients with essential hypertension.