The effects of atropine and oxprenolol on changes occurring in total catecholamine, cyclic AMP (cAMP) and lactate concentrations in arterial and coronary sinus blood, during submaximal isometric exercise, were studied in 10 patients. Static one-third-maximal handgrip exercsie, sustained for 5 minutes, did not produce an increase in either arterial or coronary sinus plasma catecholamine concentrations (measured at rest and during the last minute of exercise) and was not influenced by atropine and oxprenolol. Myocardial lactate production did not occur. Coronary sinus cAMP concentrations fell during isometric exercise from 11.53 ± 0.93 to 9.42 ± 0.81 nmol/l ( ± SEM), and following autonomic blockade from 12.46 ± 1.12 to 9.6 ± 0.87 nmol/l but rose on subsequent isometric exercise to 11.27 ± 0.8 nmol/l (p < 0.05). Although this latter increase could still be due to β-adrenergic stimulation, the absence of any change in catecholamine concentrations in the presence of β-blockade suggests that other factors may have been responsible.