The influence of concomitant food intake on the bioavailability and presystemic primary conjugation of propranolol (80 mg) was studied in 11 healthy women. Food increased the maximum serum concentration and serum AUC of propranolol (P less than 0.05) and reduced those of conjugated propranolol (P less than 0.05). The mean AUC ratio of conjugated/unchanged propranolol was 13:1 in the fasting state but only about 6:1 in the nonfasting state (P less than 0.001). The time to maximum serum concentration and the t1/2 were not affected by food. There was no influence of food on any kinetic parameter of propranolol or conjugated propranolol when a slow-release formulation was used. We conclude that concomitant food intake can evoke a short-lasting, delivery rate-dependent inhibition of the presystemic primary conjugation of propranolol. This is one, but not the sole, mechanism by which food can enhance propranolol bioavailability.