Based on studies using maximal isometric contractions, it is suggested that fatigue may be elicited due to hyperthermia-induced impairments in central neuromuscular activation. We investigated the effects of passive hyperthermia on isokinetic maximal force production. Twenty young healthy males [peak oxygen uptake 52.9 (6.8) ml x kg(-1) min(-1), body fat 11 (5)%] were passively warmed in a hot (42 degrees C) water bath to rectal temperature ( T(re)) of 39.5 degrees C and then cooled back to 38.0 degrees C. At 0.5 degrees C intervals, they performed two maximal voluntary knee extensions each at 60, 120, and 240 s(-1). Peak torques at 37.5 degrees C were 168 (34), 145 (29), and 112 (17) N m for 60, 120, and 240 s(-1) contraction speeds, respectively, and 166 (38), 150 (31), and 119 (17) N m at T(re) of 39.5 degrees C. No significant differences in peak torque at any of the three contraction speeds were observed over the range 37.5-39.5 degrees C. Skin cooling, even with a warm core of 39.5 degrees C, immediately decreased peak torque [159 (42), 133 (34), 107 (22) N m at 60, 120, and 240 s(-1), respectively). We conclude that lowered skin temperature can impair isokinetic force production independent of core temperature.