Exaggerated sympathetic activity is widely accepted as one of the fundamental mechanisms leading to primary hypertension and being responsible for frequent episodes of blood pressure elevation in hypertensive patients. Some data suggest also that basal parasympathetic tone in this entity is lowered. However, the effects of autonomic nervous activity on heart rate variability during spontaneous blood pressure elevations have not been yet evaluated. That is why we present the preliminary results of 24-hour electrocardiogram and blood pressure monitoring in 13 women with mild primary hypertension and with 25 episodes of blood pressure elevations. Time- and frequency domain measurements of heart rate variability found during the 24- and 10- hour daily periods were compared with those obtained during four 5-min records: 25-20, 15-10 min and immediately before, as well as immediately after the recording of blood pressure elevation. Significant decrease in parameters representing vagal tone was found during 5-min periods not only immediately preceding or following blood pressure elevations, but also 10 and 20 min before these episodes. Moreover, low-frequency component of heart rate variability was significantly lowered 10 min before and immediately after the recording of blood pressure elevation. These results suggest that among various pathogenetic mechanisms of spontaneous blood pressure elevations in women with primary hypertension, sudden withdrawal of parasympathetic tone should be taken into account.