It is well known that the mammalian extraocular muscle has slow muscle fiber morphologically. But the contraction speed of slow muscle fiber has not been mentioned in previous reports. We studied the isotonic contraction of slow muscle fiber and compared it with fast muscle fiber. Each fiber bundle was isolated from the rabbit superior rectus muscle. Both light microscopic findings and the reaction to isotonic contraction in Ca2+ free Ringer's solution could identify each of the two muscle fiber types. Contraction speed depended on the frequency of stimuli (40-200 Hz) in both slow and fast muscle fiber. Maximum velocities of slow and fast fiber bundles were 4.3 +/- 1.53 mm/sec and 26.9 +/- 3.48mm/sec at 200Hz, respectively. It was suggested that the relaxation of slow muscle fiber did not interfere with the contraction of the fast muscle fiber. The relation between contraction velocity and afterload showed an approximately right angle hyperbolic curve.