Conflicting data on the alterations in the maximal exercise response to beta blockade (BB) may be the result of differences in the length of time the subject has been on medication, i.e., hours vs days. The purpose of this study was to examine maximal exercise responses during acute and chronic administration of BB. Twenty-eight healthy males, 14 untrained (UT) and 14 involved in a personal training regimen (TR), performed maximal treadmill tests after 1 d and 9 d under three double-blind, randomized conditions: a placebo (PLAC), propranolol (PROP) 80 mg b.i.d., and atenolol (ATEN) 100 mg o.d. Maximal heart rate (HR), oxygen consumption (VO2), ventilation (VE), and treadmill time were significantly reduced by PROP and ATEN after an acute and chronic dose when compared to PLAC (P less than 0.05) in both groups of subjects. Maximal HR was decreased more after 1 d of BB than after 9 d of BB with both PROP and ATEN in the UT subjects and with PROP only in the TR group. VO2max, VEmax, and treadmill time were also less attenuated after 9 d of BB; however, this trend did not reach statistical significance. The nonselective beta blocker, PROP, caused greater reductions in VO2max compared to the selective beta blocker, ATEN, in both groups of subjects. These data indicate that, other than a small change in maximal HR, there is no difference in the exercise response to acute and chronic BB in normal and highly conditioned individuals.