Six healthy subjects were studied on two occasions. Graded bolus injections of isoprenaline sulphate were given intravenously and control dose-response curves were drawn for the changes in heart rate and blood pressure. In a random order each subject received an intravenous infusion of either propranolol or practolol and further dose-response curves were constructed PRE- and POST-atropine (0.04 mg/kg). Exercise tachycardia was reduced 26.1 +/- 2.7% by propranolol and this was not significantly different from the reduction by practolol (21.2 +/- 1.9%). Propranolol attenuated the isoprenaline tachycardia (dose ratio 43.7) and after atropinisation the dose ratio was not significantly altered (41.1). Practolol also attenuated the isoprenaline tachycardia (dose ratio 4.4) but after atropinisation the dose ratio was significantly increased to 8.8, though this remained significantly less than the dose ratio for propranolol. At a heart rate increase of 25 beats/min, the isoprenaline-induced control fall in mean blood pressure was 9-11 mm Hg. After propranolol administration this fall was converted to a small increase of + 2.3 +/- 1.3 mm Hg. Following practolol, however, the mean blood pressure reduction was 19.7 +/- 2.9 mm Hg. Practolol did not significantly block the isoprenaline-induced fall in diastolic pressure. The difference in potency of propranolol and practolol, demonstrated by their effect on isoprenaline induced tachycardia at doses shown to have equal effects on exercise tachycardia, is contributed to but not fully explained by the reflex withdrawal of cardiac vagal tone which occurs with cardioselective but not non-selective antagonists.