Four physically active men took part in a 6-week training program. They performed three sustained leg press contractions at 50% maximal isometric strength (MIS) to exhaustion 3-4 times a week. Before training and following performance of 50% MIS to exhaustion, muscle biopsies were obtained from m. vastus lateralis for subsequent lactate analysis on freeze-dried fiber fragments. Post-training biopsies were taken (a) after a contraction performed at the same tension level and for the same time period and (b) after an exhaustive contraction held at 50% of the post-training MIS. Prior to training, lactate concentration in fast-twitch (FT) and slow-twitch (ST) muscle fibers averaged 29.8 (range 27.9-31.4) and 28.7 (24.0-32.4) mmol X kg-1 w.w, respectively. After training the values were 16.7 (11.0-27.5) (FT) and 14.1 (6.1-26.9) (ST). Lactate concentrations following exhaustive contraction were 21.3 (16.3-27.9) (FT) and 17.8 (11.5-28.0) (ST). Thus, for all situations studied, lactate concentration was greatest in FT fibers, and this difference tended to increase with training, irrespective of whether or not the contraction was maintained until the onset of exhaustion.