Eight healthy normal subjects (19-23 years) received graded intravenous isoprenaline infusions PRE- and POST-atropine, 0.04 mg/kg i.v. The adequacy of atropinization was tested with bolus intravenous injections of phenylephrine. PRE-atropine, isoprenaline caused an increase in heart rate and systolic pressure, but a decrease in diastolic pressure with little change in mean pressure (+32.8 +/- 3.0 beats/min, +18.9 +/- 3.0 mm Hg, -13.5 +/- 1.9 mm Hg, -2.7 +/- 1.9 mg Hg respectively at isoprenaline 2 micrograms/min). POST-atropine, the increase in heart rate was enhanced, the rise in systolic pressure abolished and the falls in diastolic and mean pressures exaggerated (+47.0 +/- 2.8 beats/min, -8.9 +/- 2.9 mm Hg, -27.3 +/- 2.1 mm Hg, -21.1 +/- 1.9 mm Hg, respectively at isoprenaline 2 micrograms/min). During an isoprenaline infusion, when the heart rate and blood pressure changes are stable, there is an increase rather than a decrease in cardiac vagal tone.