Golgi tendon organs of distal tendons of rectus muscles were traced in serial transverse sections using a light microscope and their identity confirmed with an electron microscope. One or two tendon organs were present in eight of ten muscles taken from four adult rhesus monkeys and none were present in the other two. Tendon organs were unusually small averaging 270 micrometers in length and 36 micrometers in maximum width; three of them were bifid at one or both ends and the remainder were fusiform. Some lay wholly within tendon without direct attachment of muscle fibres and in the others the tip of a single felderstruktur (slow, non-twitch) muscle fibre entered the tendon organ capsule proximally and occupied the full width. Nerve terminals of most tendon organs compared with those found in other muscles with similar organelles and variety of shape. In three tendon organs with inserted muscle fibres, however, terminals with regular profiles, larger aggregations of clear vesicles and fewer mitochondria were present. The latter type of terminal and the encapsulated filderstruktur muscle fibres are features shared by the large numbers of myotendinous cylinders present in extraocular muscles and it is argued that tendon organs and myotendinous cylinders might be of common origin. If this is the case, then the overwhelming majority of myotendinous cylinders suggests that tendon organs may be an aberrant development in extraocular muscles and of little significance in the total sensory output.