Twelve subjects (three females, nine male, age 32 +/- 1.3 yr) performed tests of isokinetic muscle strength and endurance under each of three treatment conditions. The three conditions, administered in double-blind, randomized order, were single oral administrations of placebo, diphenhydramine (50 mg), and terfenadine (60 mg). Tests, separated by 1 wk, began 2 h after ingestion of the drug or a placebo. Subjects performed a velocity spectrum test (VST) consisting of peak torque measurements at five velocities (90, 150, 210, 270, and 330 degrees.s-1) and a 45 s muscle endurance test (MET) at a velocity of 180 degrees.s-1. Five serial, reciprocal contractions of the knee extensors and flexors were performed at maximal effort over the subjects' full range of motion for the VST, and as many contractions as possible within 45 s for the MET. No differences in performance measures were detected across treatment conditions. This study demonstrates that a one-time administration of either terfenadine or diphenhydramine in the dosages described does not significantly affect muscle endurance or maximal peak torque at a variety of velocities.