Ten healthy young men (21.0 ± 1.5 yr, 1.79 ± 0.1 m, 82.7 ± 14.7 kg, mean ± SD) participated in eight weeks of intense unilateral resistance training (knee extension exercise) such that one leg was trained (T) and the other acted as an untrained (UT) control. After the eight weeks of training, infusions of L-[ring-d₅] phenylalanine, L-[ring-¹³C₆] phenylalanine, and d₃ -KIC were used to measure mixed muscle protein synthesis in the T and UT legs by the direct incorporation method (FSR). Protein synthesis was determined at rest, 4 and 28 h after an acute bout of resistance exercise performed at the same intensity relative to the gain in strength before and after training. Training increased mean muscle fibre cross-sectional area only in the T leg (type I: 16 ± 10 %; type II: 20 ± 19 %, P < 0.05). Muscle protein FSR in both legs at 4 h (T: 162 ± 76 %; UT: 108 ± 62 %, P < 0.01 versus rest) with the increase in the T leg being significantly higher than in the UT leg at this time (P < 0.01). At 28 h post-exercise, FSR in the T leg had returned to resting levels; however, the rate of protein synthesis in the UT leg remained elevated above resting (70 ± 49%, P < 0.01). We conclude that resistance training attenuates the protein synthetic response to acute resistance exercise, despite higher initial increases in FSR, by shortening the duration for which protein synthesis is elevated.