The effects of exercise on lean body mass (LBM), fat mass (FM), maximal oxygen uptake (VO2max), and quadriceps (QD) strength were studied in 72 male, mildly obese (X = 38% fat) subjects (X age, 43.5 yr) randomly assigned to one of eight treatments arranged in a 2 X 4 factorial plan with exercise (EX) and non-exercise (NE) and four diets as the two factors. Exercise consisted of a 3 d/wk, 8-wk aerobics program (70-85% maximum heart rate) accompanied by a calisthenics program. LBM was determined by whole body potassium (40K), FM by subtracting LBM from total body weight, VO2max using the Wilmore-Costill method, and QD strength with the Cybex II system. Weight loss of the combined EX (11.8 +/- 0.6 kg) (X +/- SE) and NE (9.2 +/- 0.3 kg) groups was not statistically different. LBM of the EX group was unchanged (from 63.1 +/- 1.9 to 62.5 +/- 2.1 kg), whereas in the NE group it was reduced from 62.6 +/- 1.1 to 59.3 +/- 1.2 kg (P less than 0.001) accounting for 36% of total weight loss. FM loss was greater for the EX group (11.2 +/- 1.5 kg) when compared to the NE (5.2 +/- 1.6 kg) group (P less than 0.001). The EX group exhibited an increase in VO2max from 2.9 +/- 0.3 to 3.4 +/- 0.2 1 X min-1 (P less than 0.001), whereas the NE group was unchanged (3.0 +/- 0.3 to 2.9 +/- 0.4 1 X min-1 (NS].(ABSTRACT TRUNCATED AT 250 WORDS)