1 Eight healthy subjects (six male, two female, aged 18-21 years) received graded intravenous bolus injections of isoprenaline sulphate. Heart rate and intra-arterial blood pressure were continuously monitored PRE- and POST-atropine (0.04 mg/kg). 2 PRE-atropine, an increase in heart rate of 25 beats/min was produced by 2.15 +/- 0.53 micrograms of isoprenaline and was associated with a fall in mean, systolic and diastolic pressures (18.9 +/- 2.8, 17.7 +/- 3.4 and 20.4 +/- 2.3 mm Hg respectively). 3 POST-atropine, the heart rate dose response curve was shifted to the right so that the dose of isoprenaline which increased heart rate 25 beats/min PRE-atropine, produced a significantly smaller heart rate rise of 20.3 +/- 1.7 beats/min (P less than 0.001). This was associated with a shift of the blood pressure dose-response curves to the left, and larger falls in mean, systolic and diastolic pressures (30.9 +/- 2.8, 31.8 +/- 3.3, 30.1 +/- 3.3 mm Hg respectively; P less than 0.01). 4 It is concluded that there is a significant contribution from a reflex withdrawal of cardiac vagal tone, to the tachycardia produced by a bolus of isoprenaline.