A new forced-swimming apparatus for determining maximum swimming time in mice was devised for use in the evaluation of the endurance capacity of Std and ddY and CDF1 mice after various diet and drug treatments. With the apparatus, a water current is generated by circulating water with a pump in a swimming pool. A spout and suction slit were contrived to generate a constant current while the strength of the current is regulated by a valve. The decrease in the leg-kicking intervals of mice accompanying the increase in the current speed confirmed that the workload is adjustable by regulation of the current speed. Compared with the number of forelimb strokes, that of the hindlimb kicks was greater. The swimming time until fatigue was observed to decrease with increasing current speed in the two strains of mice. As biochemical indexes, the blood lactate and muscle glycogen levels corroborated the correlation between current speed and increase in workload. These results indicate that the apparatus employed in the present study is suitable for the evaluation of the endurance capacity of mice and that is useful for detecting the effects of dietary differences and drug pretreatments on this capacity.