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1. A study has been made of the effect of temperature on the resting tension and on the form of the isometric twitch of isolated rat skeletal muscle. The soleus and the extensor digitorum longus (EDL) were used as representatives of slow and fast types of muscle. The main interest is in the behaviour at low temperature, when rat muscle shows characteristics which are not found with frog muscle.

2. At higher temperatures both soleus and EDL of the rat resemble frog muscle in showing a `rubber-like' form of resting tension, i.e. the tension increases with warming: this has not previously been demonstrated in mammalian muscle. But at low temperatures (below 15° C) the converse is observed; tension then increases as temperature falls. This `cold tension' is much larger with soleus than with EDL. The `cold tension' is the converse of the `rubber-like' tension also in its dependence on length; it becomes smaller when the muscle is stretched.

3. These muscles remain excitable and give consistent twitches with transverse stimulation at temperatures down to about 3° C provided the exposure to cold is not prolonged beyond a few minutes. The amplitude of the twitch is reduced by cooling, e.g. at 3° C it is about 30-50% of that at 25° C.

4. The twitch becomes very prolonged at low temperatures. With a soleus at 3-4° C it may last 40 sec. The temperature coefficient of the rate constant of the relaxation phase is very high compared with that of frog's muscle.

5. The rate constant of relaxation in a twitch at low temperature increases when the muscle is stretched. This is the reverse of what happens at higher temperature.

6. The extreme prolongation of the twitch with cooling indicates a very large decrease in the rate of decay of the active state, and it is suggested that the increase of resting tension at low temperature is due to an accumulation of `activator'.