The aim of this study was to examine the effect of the interaction of circamensal and diurnal rhythms in temperature upon the production of maximal voluntary muscle force. Ten eumenorrheic females (mean age: 24 +/- 3 yr mean body mass: 58.4 +/- 6.9 kg) participated in the experiment at both 06:00 and 18:00h at the mid-point of both the follicular and luteal phases of the menstrual cycle. Subjects performed tasks of maximal isometric lifting strength (MILS) at knee height, and endurance time (t) for lifting 45% of MILS, upon an isometric lift dynamometer. Body temperature was elevated at 18:00h and in the luteal phase by 0.52 +/- 0.4 and 0.26 +/- 0.35 degrees C, respectively. The amplitude of the diurnal variation in temperature was blunted by 0.3 degrees C within the luteal phase. Maximal isometric performance was elevated by 8% at 18:00h in the luteal phase of the cycle (p < 0.05 interaction for MILS) but unaffected by time of day in the follicular phase. Endurance time was unaffected by time or phase (p > 0.05). It should be noted that the classic diurnal rhythm in maximal voluntary isometric muscle force may not be evident in all phases of the female menstrual cycle.