1. Isometric responses of rat inferior rectus muscle to indirect and direct stimulation were compared, and conditions were found for selective direct stimulation of twitch fibres in vitro.2. Most of the twitch fibres were qualitatively ;fast'.3. The influence of length on isometric contractions and the relation between relative load and speed of sarcomere shortening of fast-twitch fibres were determined.4. The isometric twitch contraction and half-relaxation times of fast-twitch inferior rectus fibres were only about one half of those of rat extensor digitorum longus fibres in the same conditions, whereas the force: velocity properties of these two fibre groups were virtually the same. These results show that the relation between intrinsic speed of shortening and duration of the active state of the contractile material is not the same for rat extraocular and hind-limb muscles.