We addressed the problem of the circadian changes in neural control of heart period in ambulant hypertensive subjects. A running spectral analysis of R-R variability from Holter tapes provided markers of sympathetic, i.e. low-frequency component (LF) —0.10 Hz, and vagal, i.e. high-frequency component (HF) —0.25 Hz, controlling activities for the 24-h period of the recording. Significant circadian differences were observed in LF between the two groups of subjects: during night-time rest (0300—0400 h), LF was greater in hypertensives than in normotensives (56 ± 2 and 48± 2nu, respectively; P0 < .05). Furthermore, the difference between daytime and night-time LF values was progressively reduced with increasing severity of the hypertensive state, as assessed by resting arterial pressure levels. Spectral analysis of R—R variability suggests that essential hypertension may be characterized by a reduced day—night oscillation in sympathetic activity than can be quantified non-invasively using this approach.