Short-term fluctuations in blood pressure (BP) and heart rate (HR) were analysed in a group of twelve subjects with mild hypertension. Indirect finger BP was measured by a Finapres device. The effects of an oral dose of rilmenidine (1 mg) were studied in a double-blind cross-over placebo-controlled study. Compared with placebo, rilmenidine reduced the variability of standing BP after 2 hours as estimated from the standard deviation of systolic BP which diminished by 20%. Spectral analysis of fluctuations in BP showed this reduction predominated in the mid frequency (MF) region corresponding to 10-second period oscillations, which depend on the activity of the autonomic nervous system. The average reduction in MF component for standing systolic BP was 24%. In addition the MF component for HR was reduced by 17%. This frequency domain analysis demonstrates a modified BP and HR variability profile with an acute dosing of rilmenidine. This effect being limited to the MF fluctuations in standing position orients towards an effect of the drug on the sympathetic cardiovascular control.