The masseteric nerve supplying rat deep masseter muscle was traced from serial paraffin sections. The number of myelinated axons counted in a plastic transverse section close to its entrance into the muscle was 2120. Extrafusal muscle fibre counts of rat deep masseter (71,400 fibres) and anterior digastric (5,400 fibres) muscles have been performed and were found to be 2 to 3 times higher than Hiiemae's count (1971). The number of muscle spindles in deep masseter ranged from 86 to 134 and were mostly distributed in an anterior cluster. Using glyoxylic acid-induced fluorescence, the extrafusal muscle fibres of deep masseter and anterior digastric muscles were found to be adrenergically innervated, in the respective proportions of 40% and 55%. It was found that some spindle capsules (about one third) received such innervation at their equatorial region. According to histo- eind immunohistochemical reactions, the masseter muscle is uniformly composed of type IIA fibres, except for a small number of slow oxidative fibres which were restricted to spindle-cluster areas. Three types of intrafusal muscle fibre similar to those in rat hindlimb muscle were recognized. The morphology of muscle spindles in deep masseter muscle was studied with light microscopy using serial plastic sections. The number of intrafusal muscle fibres was found to range from 2 to 7, usually one bag(_1), one bagg(_2) and 2-5 chain fibres. The lengths and diameters of the intrafusal muscle fibres and the capsules were obtained. The length of spindles ranged from 950 to 4000 µm and that of capsules from 400 to 2600 µm. The innervation of anterior cluster muscle spindles in deep masseter muscle and spindles in some hindlimb muscles of the rat was studied in teased, silver impregnated material. Analysis of the sensory innervation revealed the following unusual or unique features: a high number of afferents (1-6) innervating the sensory region of each spindle; some la axons branched to supply primary endings to two bagj fibres in two separate spindles; some other la axons branched to supply primary and secondary endings in the same spindle; and some group II axons branched to supply two secondary endings in the same spindle. Some motor axons were found to be branched to supply small end plates to bagi fibres of some spindles in the anterior cluster, and therefore it is concluded here that some cluster spindles in deep masseter muscle receive dynamic β innervation. A comparative study of the occurrence of muscle spindles in jaw- closing muscles of other vertebrates revealed that they are present in frog and absent from newt and salmon.