Five healthy normotensive volunteers (mean ± SEM age 29.6 ± 4.6 years) were infused with l-noradrenaline for a 15 min period every 3 hours for 24 hours from 0900. The concentrations of l-noradrenaline used were 0.02. 0.03, 0.05 [μg·kg−1·min−1, preceded by a saline infusion, each step lasting 5 min. The lowest preinfusion blood pressures and heart rates were at 0300 and 0600, the range being 1.45 kPa (11.6 mmHg) systolic, 1.33 kPa (10.0 mmHg) diastolic pressure, and 12.2 beats·min−1, heart rate.