The effect of submaximal exercise on the pharmacokinetics of low dose intravenous propranolol was studied in 15 healthy human subjects. There was a wide individual variation in the results for each subject and a large difference in the degree of changes with exercise. The effect of exercise on the pharmacokinetics of propranolol, a flow limited drug, is marked but variable. This phenomenon may have profound effects on patients taking the drug regularly who exercise intermittently and drug doses may have to be adjusted.