Seasonal influences on the energy balance were studied in rural Beninese women. The average body weight of 130 women showed significant seasonal changes. The lowest average weight, reached in the pre-harvest period in June, was 1.15 +/- 1.27 kg lower than the average weight in the post-harvest period in August. This response was not uniform however: while well-nourished women with a BMI of more than 23 at the beginning of the study showed a marked weight loss in the pre-harvest period, thin women with a BMI of less than 18 did not. Energy intake was studied in a sub-group of 18 women and found to vary between 1550 kcal/day (6485 kJ/day) in May-June to 1900 kcal/day (7950 kJ/day) in March-April. In the pre-harvest period there was a reduction in physical activity. BMR (n = 17) remained at a constant level of about 1330 kcal/day (5556 kJ day) throughout the year.