The effect of lowering temperature on the resting tension of frog's striated muscle was studied at various sarcomere lengths, using whole muscle, single-fibre and skinned-fibre preparations. And the temperature coefficient of tension, beta=(1/P)(dP/dT) (P, tension; T, temperature) was investigated. In whole sartorius muscles in Ringer solution, beta was negative over the whole range of sarcomere lengths investigated. In the presence of 2 mM procaine the rubber-like thermoelasticity (beta greater than 0) was revealed at medium lengths. At shorter (greater than 2.3 mum) and longer (greater 2.7 mum) lengths, beta stayed negative. In single muscle fibres suspended in Ringer solution with mM procaine, beta was positive over a wider range of lengths (2;3-3.4 mum), and the peak positive value of beta was obtained at the length of 2.6 mum. In skinned fibres suspended in a relaxing solution, beta stayed positive up to the length of 4.2 mum, and the peak value of beta was obtained at the length of 3.0 mum which was considerably longer than in single fibres. The results substantiated A. V. Hill's view on the thermoelasticity of the muscle; there are components with positive beta (contractile apparatus) and those with negative beta (connective tissue and sarcolemma), and at high elongations the positive coefficient is masked by the increasing importance of the negative coefficient.,