The purpose of this study was to determine whether aging alters changes in resting metabolic rate, body composition and insulin sensitivity in response to resistance training (RT) in women. The impact of a 6-month controlled RT program on fat-free mass (FFM), resting energy expenditure (REE), and glucose disposal was examined in 19 younger (27.8+/-3.5 yr; range 18-35) and 12 older (66.6+/-4.9 yr, range 55-70) non-obese caucasian women. For younger women, the RT program increased body weight (59.1+/-6.2 to 60.8+/-6.1 kg, p<0.05) due to an increase in FFM (39.2+/-3.7-40.4+/-3.2 kg, p<0.05). REE increased from 1379+/-114 to 1451+/-140 kcal day(-1), p<0.0001), and glucose disposal (from 364.1+/-91.1 to 402.1+/-87.8 mg min(-1), p<0.05). Neither fat mass nor VO2max changed significantly. On the other hand, older women lost fat mass (24.0+/-4.6-23.2+/-5.4 kg, p<0.05) and tended to gain FFM (from 37.3+/-4.0-38.0+/-4.3 kg, +1.9%; p=0.057), whereas no change occurred in body weight, REE, VO(2)max or glucose disposal. Thus, younger women showed greater metabolic changes in body composition, REE and insulin sensitivity in response to RT than older women. These results suggest an age-related alteration in metabolic changes in response to a 6-month RT program in previously untrained women.