The aim of this study was to compare changes in aerobic fitness and body composition in response to British Army (regular) and Territorial Army (reserve) basic training. Eleven regular recruits, 14 reserve recruits, and 20 controls completed the study (all males). Initially, reserve recruits were significantly older and heavier and had greater fat-free mass (FFM; 64.6 vs. 59.3 kg) and lower maximal oxygen uptake (Vo(2)max; 39.1 vs. 43.9 ml.kg(-1).min(-1)) than regular recruits. Both regular and reserve training significantly increased FFM and Vo(2)max and decreased percentage body fat. Regular training produced a greater increase in Vo(2)max than reserve training (13.1 vs. 7.6%, p < 0.0005). Reserve training produced a greater increase in body mass (2.2 vs. 0.9 kg, p = 0.019) and tended to produce a greater increase in FFM (2.6 vs. 1.6 kg, p = 0.062). Although both training programs improve aerobic fitness and body composition, increasing the volume of physical training in the reserve training program would probably enhance the training adaptations achieved.