MONTGOMERY, L. C. and P. A. DEUSTER. Ingestion of an antihistamine does not affect exercise performance. Med. Sci. Sports Exerc., Vol. 24, No. 3, pp. 383–388, 1992. The effects of a single oral dose of a sedating and nonsedating H1 receptor antagonist on exercise performance and tolerance were examined in a double-blind, randomized study. Twelve healthy, physically active subjects were tested under a placebo condition and two antihistamine conditions (diphen-hydramine hydrochloride (50 mg) and terfenadine (60 mg)). The following treadmill exercise tests were administered: maximal aerobic power (MAX), submaximal steady state (SS), and high-intensity, intermittent exercise (HI). The MAX test consisted of incremental treadmill running to volitional exhaustion. For the SS test, subjects ran for 30 min on the treadmill at approximately 55% of their V̇O2max. The HI test consisted of alternated 30-s bouts of running and rest to volitional exhaustion; the treadmill grade was 10% and the intensity was nearly 90% of V̇O2max. Measures of oxygen uptake, heart rate, rectal temperature, post-exercise plasma lactate concentration, and time on treadmill for the MAX and HI tests were compared across treatment conditions. There were no statistically significant differences in performance measures across treatment conditions. The results indicate that a single dose of antihistamine will neither compromise nor improve aerobic and glycolytic work performance under these exercise conditions.