The validity of using the basal metabolic rate (BMR) to calculate an individual's energy requirements is based upon the assumption that the intraindividual variation in BMR is small. Early studies (pre-1940) on BMR in women had shown that the menstrual cycle may have a profound effect, contributing to high levels of intraindividual variation. To investigate this issue further, and to explore whether BMR is indeed a biological constant in women, sequential measurements of BMR were made in women over one menstrual cycle. Two independent studies were undertaken in which the BMR of 12 weight-stable women (not taking the contraceptive pill) was measured every day (excluding weekends) for a period of 5 weeks. The six women participating in the first study were measured using a Douglas bag, while the six subjects in the second study were measured with a Deltatrac (Datex, Helsinki). Nine of the 12 subjects demonstrated a peak in BMR during the late luteal phase of the menstrual cycle, while 8 of the 12 subjects exhibited a fall in BMR after the onset of menstruation. Group analysis of the results indicated that the BMR during the early follicular phase was significantly lower than the BMR during the late luteal phase (Wilcoxon's signed rank test: P < 0.01). The level of intraindividual variation was assessed by calculating the coefficient of variation (CV) for the measurement period. The CV in six of the women exhibited a level of variation comparable to men (2-4%). However, intraindividual variation in the BMR of the other six women was considerably higher (up to 12%). In these six women, therefore, BMR may not be considered a biological constant. All available data on sequential measurements of BMR in fertile women were collated from the literature for the first time, and statistically analyzed. BMR during the early follicular phase was found to be significantly lower (P < 0.05) than during the late luteal phase. Further work on the biological constancy of BMR in women and the impact of the menstrual cycle on BMR is required. This area is of particular importance given the fact that the energy requirements of both individuals and populations are now calculated using the BMR. Β© 1996 Wiley-Liss, Inc.