The effect of temperature (5-35 ° C) on maximum force production was examined in intact and chemically skinned muscle fibre bundles (10-25 fibres) from the anterior byssus retractor muscle of Mytilus edulis. In intact fibre bundles, 10 ~M acetylcholine induced a tonic contraction which had a magnitude of 65.4 + 4.0 N cm -2 (n = 30) at 23 ° C. Activation by caffeine (20 mM) produced a force response which was 157.1 + 7.9% (n = 16) of the acetylcholine response at 23 ° C and acetylcholine and caffeine together produced force which was not significantly different from the response to caffeine alone. At 5 ° C the acetylcholine and caffeine responses were decreased by 9.6 + 3.4% (n = 6) and 14.6 + 2.8% (n = 8) compared with the respective responses at 23 ° C. However, there was no significant reduction of the response induced by the combined action of acetylcholine and caffeine when the temperature was decreased from 23 ° C to 5 ° C. The 20-80% of peak force activation time increased by about one order of magnitude for all acetylcholine, caffeine and combined acetylcholine-caffeine-induced responses when the temperature was decreased from 23-5 ° C. Repeated exposure of the intact preparation to caffeine caused a marked decrease in the caffeine-induced response (complete abolition of force after the third exposure to caffeine), but the response to caffeine could be fully restored following one acetylcholine-induced activation. The maximum Ca2+-activated force after skinning the preparation with saponin was not significantly different from the caffeine or combined acetylcholine-caffeine-induced responses before skinning. In the saponin skinned fibre preparation a drop in temperature from 23 ° C to 15 ° C or 5 ° C decreased the maximum Ca2+-activated force by 13.2 _ 1.4% (n = 8) and 41.4 + 3.1% (n = 5) respectively. The activation time between 20-80% of the peak Ca2+-activated force increased at 15 ° C and 5 ° C by a factor of 1.5 + 0.1 (n = 5) and 6.8 + 1.1 (n = 5) respectively when compared to corresponding values at 23 ° C. The relaxation half-time decreased by a factor of 1.7 + 0.2 (n = 5) and 3.0 + 0.2 (n = 5) at 15 ° C and 5 ° C respectively compared with that at 23 ° C. It was possible to distinguish between the temperature effects on the contractile apparatus per se and the Ca 2+ regulatory system with the results indicating that the contractile apparatus was more sensitive to a change in temperature than the Ca2+-regulatory system. Increasing the temperature to 35 ° C irreversibly affected the ability to develop and maintain force in both intact and skinned muscle preparations. These results indicate that: (1) acetylcholine does not fully activate the intact 'catch' muscle at 23 ° C; (2) acetylcholine is able to replenish the internal stores after depletion by caffeine; (3) compensatory mechanisms which oppose the inhibitory effect of lower temperatures on the contractile apparatus and the Ca2+-regulatory system must be operating in the intact fibre preparations.