At a muscle length, L0 (just taut), isometric tension and tension transients in response to rapid step stretches in length (less than 1 per cent of L0 within 2 ms) were measured at constant levels of Ca2+ activation of various magnitudes in glycerinated temporal and masseter muscles (1.7-2.5 mm long, 48-96 micron in diameter) from guinea pigs. The experiments were at 20 and 30 degrees C, and pH 6.8. Steady isometric tension increased in a sigmoid fashion as Ca2+ varied from about pCa 6.6 to 4.4 in both muscles. However, the maximum tension of temporal muscle was about 4.4 times greater than that of masseter muscle at 20 degrees C. The tension transients showed an immediate tension increase coincident with the stretch (the first phase) and an exponential-like tension decrease (the second phase). The time constant of the second phase was about 87.5 ms in temporalis and 3.7 ms in masseter at 30 degrees C. Decreasing temperature from 30 to 20 degrees C markedly increased the time constant of the tension response in the second phase (Q10 of about 3.4 in temporalis and 2.2 in masseter). Although there was a difference in the sarcomere length between the two muscles at L0 (2.14 micron in temporalis and 1.84 micron in masseter), the mechanical characteristics were almost independent of the muscle length. Thus the cross-bridges in temporalis appear to cycle more slowly and produce more isometric tension than those in masseter muscle.