The purpose of the present study was to investigate the effects of 3-month sprint and endurance training programs on the vastus lateralis muscle fiber area and the activities of glycolytic (phosphofructokinase; PFK) and oxidative (succinate dehydrogenase; SDH) enzymes of adolescent boys. Enzyme activities were also determined after a subsequent 6-month detraining period. Endurance training resulted in significant increases in VO2max (58.4 to 64.3 ml . min-1 . kg-1), in ST and FTa fiber area (6.0 to 7.3 and 8.0 to 10.4 microns 2 x 10(3), respectively), and in SDH activity (6.4 to 9.1 IU). After detraining VO2max and SDH activity returned to pretraining levels. Sprint training resulted in a significant increase only in PFK activity (28.1 to 33.9 IU), which was also abolished in the detraining period. These data demonstrate that in adolescent boys skeletal muscle enzyme changes are specific to the mode of training and that they are similar in direction but different in magnitude to those found in adults.