Muscle force production and power output in active males, regardless of the site of measurement (hand, leg, or back), are higher in the evening than the morning. This diurnal variation is attributed to motivational, peripheral, and central factors and higher core and, possibly, muscle temperatures in the evening. This study investigated whether decreasing evening resting rectal temperatures to morning values, by immersion in a water tank, leads to muscle force production and power output becoming equal to morning values in motivated subjects. Ten healthy active males (mean ± SD: age, 22.5 ± 1.3 yrs; body mass, 80.1 ± 7.8 kg; height, 1.72 ± 0.05 m) completed the study, which was approved by the local ethics committee of the university. The subjects were familiarized with the techniques and protocol and then completed three sessions (separated by at least 48 h): control morning (07:30 h) and evening (17:30 h) sessions (with an active 5-min warm-up on a cycle ergometer at 150 W) and then a further session at 17:30 h but preceded by an immersion in cold water (~16.5 °C) to lower rectal temperature (Trec) to morning values. During each trial, three measures of grip strength, isokinetic leg strength measurements (of knee flexion and extension at 1.05 and 4.19 rad s(-1) through a 90° range of motion), and three measures of maximal voluntary contraction (MVC) on an isometric dynamometer (utilizing the twitch-interpolation technique) were performed. Trec, rating of perceived exertion (RPE), and thermal comfort (TC) were also measured after the subjects had reclined for 30 min at the start of the protocol and prior to the measures for grip, isokinetic, and isometric dynamometry. Muscle temperature was taken after the warm-up or water immersion and immediately before the isokinetic and MVC measurements. Data were analyzed using general linear models with repeated measures. Trec values were higher at rest in the evening (by 0.37 °C; p < 0.05) than the morning, but values were no different from morning values immediately after the passive pre-cooling. However, Trec progressively decreased throughout the experiments, this being reflected in the subjects' ratings of thermal comfort. Muscle temperatures also displayed significant diurnal variation, with higher values in the evening (by 0.39 °C; p < 0.05). Right grip strength, isometric peak power, isokinetic knee flexion and extension for peak torque and peak power at 1.05 rad s(-1), and knee extension for peak torque at 4.19 rad s(-1) all showed higher values in the evening (a range of 3-14%), and all other measures of strength or power showed a statistical trend to be higher in the evening (0.10 > p > 0.05). Pre-cooling in the evening significantly reduced force or power variables towards morning values. In summary, effects of time of day were seen in some measures of muscle performance, in agreement with past research. However, in this population of motivated subjects, there was evidence that decreasing evening Trec to morning values by coldwater immersion decreased muscle strength to values similar to those found in the morning. It is concluded that diurnal changes in muscle performance are linked to diurnal changes in Trec.