Kittens were raised with amblyopia by monocular lid suture or esotropia by transection of extraocular muscles in one eye at the age of 2--3 weeks. Isometric contractions in the inferior oblique muscle were recorded at 20 weeks or at adult age, and the results were compared with those of similarly aged cats with normal visual development. The speed of contraction and the fatigue resistance was reduced in the lid sutured animals. In the esotropic cats studied, fatigue resistance was also reduced but the speed of contraction did not change much. It is suggested that the postnatal eye muscle development can be modified by manipulation of the visual input at an early age. Presumably the impaired binocular vision in the lid sutured and esotropic cats reduced the demand for fusion vergences, and this might be reflected in the changes of eye muscle properties. In monocularly lid sutured cats, eye muscle changes were the same on both sides, suggesting that amblyopia per se did not affect eye muscle development.