The 24-hour variability in blood pressure and heart rate of 14 normotensive subjects was examined under ambulatory conditions with a continuous, invasive method. The results were analysed with 30-second means calculated from beat-to-beat values. The registered time series had three different types of variation: (1) a continuous, fast variation around the mean level; (2) a stepwise, permanent change to a new level; and (3) a slow, continuous rise or fall. Variability in blood pressure and heart rate was determined by the 80 percent range of variation calculated from the cumulative distribution as the difference between the 90th and 10th percentiles. The variability in blood pressure and heart rate was significantly greater during waking hours than during night-time. Variability followed a similar type of diurnal profile as the mean level. However, variability of diastolic pressure changed to a lesser extent than that of systolic pressure. Total 24-hour variability was on average 45 mmHg in systolic blood pressure, 24 mmHg in diastolic pressure and 40 b.p.m. in heart rate. Using night-time to describe the basal state, physical and mental activities in the evening and during daytime were found to raise variability in systolic pressure with 66%, diastolic with 8-16% and heart rate with 130-180%, respectively. The results show that in normotensive subjects the sleep-wake state and activity are the major factors that affect both the diurnal profile and the variability in blood pressure and heart rate.