The effects of an 8-week swim training program and a 3-week detraining period on the size and protein composition of the heart of female Wistar rats were examined. The animals were separated into three groups: trained, detrained, and sedentary control. The training program, 6 hours/day, 5 days/week, resulted in a significantly larger dry heart weight, a normal collagen concentration, and a nonsignificantly greater total protein content. The greatest changes occurred between 4 and 6 weeks of training. The mild physiologic hypertrophy was assumed to be typical, and therefore simply an enlargement of a normal heart with proportional increases in myocyte size, and connective and vascular tissue hyperplasia. The detraining period produced a return of the heart mass to normal by the end of 2 weeks of detraining. The total protein content and collagen concentration remained elevated, though nonsignificant. The results indicate the detrained heart is composed of more connective, vascular, and metabolically active tissue, all of which may be important in the detrained heart’s response to increased work.Ball State UniversityMuncie, IN 47306