METHODS

Animals and training program. Male Wistar rats weighing approximately 250 g at the beginning of the study were randomly assigned to one of three groups. These were I) sedentary cage-confined rats, 2) rats subjected to the swimming program of 75 min twice daily, 5 days/wk for 8 wk, that has been used in previous studies from this laboratory, and 3) rats made to run on a motordriven treadmill (Quinton model 42-15) for similar durations as the swimming group but at a speed and incline that were adjusted to approximate the intensity of running to that of s wimming. A period of 4 h was allowed between the daily training sessions. Swimming rats were able to attain their full exercise program by the 2nd wk, whereas the running rats required 4 wk to reach this level. However, both groups were performing their complete exercise load for a period of at least 4 wk before they were studied. All animals were kept in the same room and housed five animals to a cage. They were fed commercial rat chow and watered ad lib&urn. To exercise the animals at similar intensities the oxygen cost of exercise was estimated from the studies of McArdle (14) and Shepherd and Gollnick (23). According to the data of McArdle, rats swimming with no load attached to their tails have an oxygen consumption that is approximately 75% of their maximal value. Data interpolated from Shepherd and Gollnick would indicate that rats running at 20 m/min up a slight (8”) incline would achieve a similar percentage of maximal oxygen con-sumption. In both of the studies cited, the rats had been