When applying various concentrations of NaCLO4 (0.15–7 m) to tendon fibres from rat tails, we found denaturation beginning at 2 m, and with increasing concentration a linear increase of the maximal contraction tension and an exponential shortening of the contraction time down to a minimal value of 7.5 min. In a second group of experiments we investigated the weight changes of tendons of old (26–30 months) and young (8 months) rats treated with 5 m NaCLO4 and compared the weight curves and the tension curves. We found an increase in weight up to 900% as the tension decreased. This indicates a connection between the swelling and the relaxation of collagen fibres. Other solutions of neutral salts (5 m NaCLO3, NaCl; 15 m NaCLO4, NaCLO3, NaCl) tested on young and old tendons did not produce any tension nor other visible changes. The weight curves show an immediate increase up to about 200% which remains constant and which is low compaired with the effect of 5 M NaCLO4. The reason for the contraction of tendon fibres seems to be the denaturation caused by the increased lipophilia of the aqueous salt solutions, while the relaxation is merely the result of the subsequent swelling of the collagen fi