The purpose of this study is to observe the applicability of endurance training at young age. ICR strain mice were classified into 14 groups; 5 control and 9 swimming training groups. 15T, 19T, 23T, 32T and 38T groups started a 12 week training at 3, 7, 11, 20 and 26 weeks after birth, respectively. 29-T, 25-T, 21-T, 12-T (32T) and 6-T groups started the training at 3, 7, 11, 20 and 26 weeks after birth, respectively and subjected to swimming training till 32 weeks. After the training period, contractile properties of the tibialis anterior muscle were investigated.
　As for the groups with the same training period, endurance capacity increased as the training started later. At the same age of 32 weeks, earlier start of training resulted in a decrease in the rate of tension development. Rates of tension decrease during tetanic contraction for 15 sec were not related to the length of training period and were lease as the starting period of training was delayed.
　These results suggested that pre-maturity period may not be available for starting endurance training in the course of postnatal growth in mice.

DESCENTE SPORTS SCIENCE Vol.8/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE