This study investigated whether 30 min of acute hot exposure has an additional passive warm-up effect for the improvement in muscle performance in a moderately warm and humid environment. We also sought to determine whether this effect is dependent on the diurnal variation in body temperature. Nine male subjects (age: 31.9 [+/- 5] years, height: 177 [+/- 6] cm, body mass: 69.3 [+/- 10] kg) were tested (CMJ, cycling sprints, and isokinetic contractions of the knee flexors and extensors) in a moderately warm and humid environment (24 [+/- 1] degrees C and 70 [+/- 4] % rh) with and without acute heat exposure (30 min of rest in a sauna at 76 [+/- 2] degrees C and 27 [+/- 1] % rh), both in the morning (07:00 - 09:00 h) and in the evening (17:00 - 19:00 h). Our results indicated a significant effect of both time-of-day and acute heat exposure on leg skin temperature (p < 0.01) but failed to show any effect of time-of-day or acute heat exposure on the various evaluated parameters (CMJ: speed, force, power and height; cycling power: over a half pedal revolution and a total pedal revolution; isokinetic torque: knee extensor and flexor muscles at 4.19 rad x s (-1), 3.14 rad x s (-1), 2.09 rad x s (-1), and 1.05 rad x s (-1)). In conclusion, our data suggest that 30 min of acute hot exposure does not have any passive warm-up effect in a moderately warm and humid environment. Furthermore, the diurnal variation in body temperature has no passive warm-up effect in a moderately warm and humid or in an extremely hot environment.