The variation of lengths, cross-sectional areas, weights, tetanic tensions, and passive tensions in extensor digitorum longus muscles in mice were investigated during postnatal growth. The increase of tetanic tension was discussed in relation to morphological changes of muscle. The muscle lengths were measured at right angle of articulatio genus and talocruralis (L0) . Following results were obtained: 1) The length, cross-sectional area and weight of muscle increased rapidly up to 7 th weeks, and tetanic tension developed significantly in 2 nd to 5 th week. 2) The muscle length (ML), muscle cross-sectional area (MCA) and muscle weight (MW) correlated significantly with the measured tetanic tension (TT) at L0 (the correlation coefficient r=0.852, 0.886 and 0.903 in ML, MCA and MW respectively) . 3) TT/MCA and TT/MW decreased in 7 th to 8 th week. 4) The passive tension at 130% L0per MCA showed maximum values in 8 th and 9 th weeks. 5) Relative muscle endurance were reduced in 2 nd to 4 th week and reached to steady level after 4 th week. These observations suggest that the functional development of muscle is late for morphological one and/or rapid morphological increment affects the contractile properties of muscle.