Small length changes were imposed on pairs of sartorius muscles from Rana temporaria and Rana pipiens in rigor and the mechanical and thermal responses studied. Rigor was induced by soaking the muscles overnight at 0 degrees C in a physiological salt solution containing 1.5 mM sodium azide and 0.4 mM sodium iodoacetate. Tension was measured at both the tibial and the pelvic ends of the preparation. Muscles were held at a steady tension of 20 to 76 kN m-2 and stretches or releases of 0.02 to 0.6 mm applied in pairs, with the initial change reversed several hundred milliseconds later. Single stretches resulted in heat absorption and releases in heat production by the preparation. Net heat production resulted from complete cycles of length changes larger than 0.1 mm, whether the initial change was a stretch or a release. The heat produced by the complete cycle was attributed to the movement of the muscles over the thermopile. It was proportional to the difference in tension between the tibial and pelvic ends of the preparation and increased with the size and speed of the length change. Half the heat produced by a complete cycle of length changes was subtracted from the thermal response recorded in the first half-cycle to obtain the reversible component of the response. The reversible component was linearly related to the tension change for all sizes and speeds of length change which were studied, with the heat:tension ratio ranging from -0.0093 to -0.0179 in eleven muscles (mean -0.0128 +/- 0.0009). The constancy of the heat:tension ratio in rigor muscles over a wide range of mechanical conditions indicates that the source of the thermal changes is the normal elasticity of the preparation. Since the size of the ratio is approximately the same as that measured in active muscles, the tension-dependent component of the thermal response to length changes applied to active muscles is probably also of elastic origin.