1. The effect of twice-daily dosing with propranolol on the pharmacokinetics and pharmacodynamics of a single oral dose of sumatriptan was investigated in 10 healthy male subjects. 2. Each subject received 7 days dosing with propranolol (80 mg twice daily) plus a single dose of sumatriptan (300 mg orally) on day 7; on another separate occasion, placebo was administered for 7 days plus a single dose of sumatriptan on day 7. There was at least a 7 day washout interval between the two periods of dosing. Pulse and blood pressure were measured up to 10 h after dosing with sumatriptan and blood samples were taken up to 26 h post-dose. 3. Propranolol had no significant effect on any of the derived pharmacokinetic parameters of sumatriptan. The appropriate average parameter values in the presence of propranolol were, respectively: Cmax (120 ng ml-1 vs 126 ng ml-1), tmax (4.5 h vs 3.0 h), AUC (580 ng ml-1 h vs 566 ng ml-1 h), t 1/2,z (1.9 h vs 1.8 h). 4. Propranolol had no significant effect on the pharmacodynamics of sumatriptan, as measured by pulse rate and blood pressure. 5. The results of this study would suggest that no alteration in the sumatriptan dosage will be necessary for migraine patients taking propranolol prophylactic therapy.