In three monkeys, the right eye was moved from the primary position to a position of 30 degrees to 45 degrees exotropia by suturing the globe to the orbital wall. In two animals perfused immediately following suturing, histological examination showed the medial rectus and its sarcomeres to be lengthened and the lateral rectus and its sarcomeres to be shortened as compared to those of the unoperated control eye. In the third monkey, after exotropia was maintained for 2 months, the muscle lengths changed but the sarcomere lengths were similar to those of the control eye. We interpret this to indicate addition of sarcomeres to the lengthened medial rectus and removal of sarcomeres from the shortened lateral rectus. This adaptation to conform muscle length to eye position may explain the altered eye position which persists following periods of eye deviation due to muscle paralysis, prior surgery, injury, or botulinum toxin treatment.