1 The effect of single oral doses of six beta-receptor antagonists on exercise-induced changes in double product (systolic blood pressure x heart rate) were studied in 25 human volunteers. 2 Three doses of propranolol, nadolol, oxprenolol, pindolol, timolol and atenolol were selected for study on the basis of in vivo beta-blocking potency. 3 Although all beta-blockers studied reduced the double product response to exercise, the pharmacodynamics of this effect differed markedly. 4 Pharmacodynamic half-lives, estimated for the drug tested, were 39 h for nadolol, atenolol 21 h, timolol 15 h, oxprenolol 13 h, propranolol 11 and pindolol 8 h. 5 These results suggest that the clinical choice of a beta-blocker with the least problems of compliance can be made on the basis of pharmacodynamics as well as pharmacological profile.