Study aim: To assess the effects of 8-week endurance training in swimming on work capacity of boys aged 12 years.Material and methods: The following groups of schoolboys aged 12 years were studied: untrained control (UC; n = 14) and those training swimming for two years. The latter ones were subjected to 8-week training in classical style (CS; n = 10) or free style (FS; n = 13). In all boys maximal oxygen uptake (O2max) was determined, and the CS and FS groups were subjected to 6 tests: swimming at 50 and 400 m distances (time recorded) and to 12-min swimming (distance recorded), all by free and classical styles pre- and post-training. From swimming times at 50 and 400 m distances the so-called critical swimming speed (CSS) was computed: CSS = (400 - 50) / (t400 - t50).Results: No training-induced improvement in O2max was noted in any group. Yet, boys subjected to classical style training significantly (p<0.05) improved their free-style swimming velocity at CSS and at the 400-m distance by about 6%, and their heart rate following the 12-min test in classical style decreased by nearly 16% (p<0.001) compared with the pre-training values.Conclusions: The 8-week training in given swimming style does not negatively affect the performance in other style than the trained one. This may be of importance in competitive training.