The effects of acute exercise (swimming) on food intake (FI) and plasma free fatty acids (PFFA) were examined in male adult rats. FI decreased during the first hour after swimming; it was unchanged during the second hour; it decreased again during the third hour. Body weight decreased over the 24 hours following exercise (-1.7 +/- 1 g) while the animals gained weight during no exercise periods (+3.2 +/- 1 g). At the end of exercise, plasma glucose augmented as well as PFFA levels. An increase in sympathoadrenal activity per se and its inhibitory effect on insulin release can both be responsible for fatty acid mobilization from adipose tissue. Lipolysis could be a cause of hypophagia.