The purpose of this investigation was to determine whether the concept of the critical power could be applied to competitive swimming by using critical swimming speed (CS) as determined both in the swimming flume (CS-flume) and in the normal swimming pool (CS-pool) and whether CS could be utilized as a practical index for assessing a swimmer's endurance performance. CS defined as the swimming speed which could be theoretically maintained continuously without exhaustion was expressed as the slope of a regression line between swimming distance (D) and its duration (T) obtained at various swimming speeds. Eight highly trained swimmers were instructed to swim until onset of fatigue at four predetermined swimming speed levels in the swimming flume and at maximal effort over four different swimming distances in the swimming pool. In the results of CS-flume and CS-pool, the regression relations between D and T were expressed in the general form, D = a+b x T, with r2 being higher than 0.998 (p less than 0.01), respectively. These results both from the flume and the pool indicated extremely good linearity. Furthermore, maximal oxygen uptake (VO2max) during the incremental exercise test, swimming speed corresponding 4 mM of blood lactate concentration (V-OBLA) and mean velocity in the 400 m freestyle (V-400) were measured on each subject.(ABSTRACT TRUNCATED AT 250 WORDS)