Eighty Sprague-Dawley rats were divided into exercised (E) and sedentary (S) groups. E rats were trained to run on a treadmill (30 m/min, 2 hr/day). Within each group, two subgroups were mated and three served as virgin time controls. Of the mated subgroups, one was terminated within 24 hours of delivery and the other on day 14 of lactation. Subgroups of virgin S and E controls were terminated at times corresponding to the mating, delivery and lactation day 14 of mated rats. MANOVA revealed that exercise significantly affected food intake, body weight and body composition in both virgin and mated animals: generally, E rats ate more, gained more weight, and had less carcass fat than S controls. E rats did not store fat during pregnancy. At parturition, they were 7.0% fat, similar to both E (6.6%) and S (7.6%) controls prior to mating, and less than S rats at parturition (11.9%). Despite diminished fat stores at parturition in E rats, litter size and pup birthweight were similar in E and S rats, as was offspring growth during lactation (mean weights on day 14 of 28.9 g and 29.3 g, respectively). Remaining body fat and increased food intake were adequate to support normal pup growth.