Post-exercise ketosis is not abolished by glucose ingestion immediately after exercise but is counteracted by simultaneous beta-adrenergic blockade. To investigate the effect of beta-adrenergic blockade on post-exercise ketosis without the ingestion of glucose, we administered propranolol (1 mg/kg body mass) to 15 carbohydrate-starved people, of whom five had just walked 9 km in 2 h. There were 43 control subjects (no propranolol). The blood concentration of 3-hydroxybutyrate rose from 0.18 +/- 0.02 (S.E.M.) mmol/l at 07.00 h to 0.35 +/- 0.04 mmol/l at 09.00 h whether the subjects had exercised during those 2 h or not (d.f. = 57). The blood concentration of 3-hydroxybutyrate at 15.00 h in the groups not treated with propranolol was not affected by exercise (0.95 +/- 0.90 mmol/l; d.f. = 42). Propranolol significantly raised the concentration of 3-hydroxybutyrate at 15.00 h to 1.68 +/- 0.26 mmol/l when given after exercise (d.f. = 4), but lowered it to 0.46 +/- 0.07 mmol/l in the non-exercised group (d.f. = 9). This was not accompanied by significant differences in the blood concentrations of glucose, free fatty acid, insulin or glucagon. The difference in response to propranolol administration is probably determined by the alanine and lactate flux from muscle for hepatic oxaloacetate synthesis.