Two elite junior ice hockey teams with players ranging in age between 16 and 20 years were measured on an extensive battery of laboratory tests at the beginning and end of a season in order to determine adaptive changes in the energy supply systems and associated functions. Pre- to post-season changes in maximal aerobic power showed a significant increase of 3% (p <.05) only when expressed in 1/min (4.30 vs 4.43). Additionally, when measurements were made on selected determinants of oxygen transport, no change was found in ventilation as reflected in VC (5.60 vs 5.52 /min), FEV1.0(4.37 vs 4.44 1/min), MBC (185.1 vs 186.4 1/min) and maximum exercise VE (128.0 vs 128.1 1/min, STPD) or in the cardiovascular system as reflected in hematocrit (45.3 vs 44.4%), hemoglobin (16.4 vs 16.2 g%) and maximum exercise HR (192 vs 191 beats/min). On the other hand, indications of change in the anaerobic energy supply systems were found. Maximal anaerobic capacity improved by 16.3% (p < 0.05) when measured in terms of run time (64.3 vs 74.8 sees) and maximal anaerobic power by 4.7% (p < 0.05) when expressed in m/sec (1.69 vs 1.77). Although comparable percentage increases were noted when peak lactate concentration and power (kgm/sec) were used as criterion measures for these tests respectively, statistical significance was not found. When the results were analyzed by position essentially the same changes were found.