To clarify whether a circadian rhythm of peripheral resistance exists in humans and whether hypertensive patients represent a homogeneous category in this respect, 15 normotensives aged 31 +/- 4 years and 30 hypertensives aged 41 +/- 13 years were confined to bed for 22 h and forearm flow recorded automatically. Night-time BP values were higher in hypertensive patients (Group B) whose night/day ratios of mean BP were below the 95% C.I. of the normal regression of the normotensives, than in those falling within the 95% C.I. (Group A). Forearm resistance was lower during sleep than during waking in Group A and in the normotensive controls, paralleling the nocturnal blood pressure fall. On the contrary, in the Group B hypertensives, despite a comparable night-time BP decrease, forearm resistance was higher during sleep than during waking.