Experiments were addressed to the following questions: when a muscle nerve is sectioned and regenerates to what extent are muscle receptors (spindles and tendon organs) reinnervated? is the reinnervation specific? that is, do group Ia and spindle group II fibres preferentially reinnervate spindles and do group Ib fibres preferentially reinnervate tendon organs? what are the consequences to the afferent of failure to re-establish appropriate receptor innervation? In normal cats, and in cats 3, 6 or 9 months after section and resuture of the medial gastrocnemius muscle nerve, medial gastrocnemius afferent fibres in continuity were impaled in dorsal rootlets for recording and stimulation. Receptor innervation was determined electrophysiologically by manipulation of the medial gastrocnemius muscle. Afferent fibre type was determined by the presence (group Ia or spindle group II) or absence (group Ib) of field potentials in the homonymous motoneurone pool in response to activation of the afferent fibre. In normal cats, two-thirds of recorded afferents innervated spindles; 89% of these generated field potentials detected in the motoneurone pool. One-third of recorded afferents innervated tendon organs; none of these generated such field potentials. In operated cats, about half of the recorded afferents innervated spindles, about one-third responded abnormally or not at all to muscle manipulation, and fewer than one-tenth innervated tendon organs. Numbers of afferents which innervated spindles increased with time. The proportion of afferents generating field potentials was smaller in operated than in normal animals (40% vs. 60%) and declined progressively with time. Field potentials were generated by fibres in all categories of receptor reinnervation. This ability was lost at long post-operative intervals by fibres failing to reach the muscle. Conduction velocity of fibres fell in operated animals. Fibres reinnervating their original type of receptor (e.g. group Ia fibre----spindle) exhibited greater conduction velocity than fibres innervating an inappropriate receptor or no receptor. From these findings and other considerations (see Discussion) we conclude that following section and resuture of the medial gastrocnemius muscle nerve: about 75% of afferents regenerate into the medial gastrocnemius muscle, many more spindles than tendon organs become reinnervated, random populations of groups Ia and Ib (and probably spindle group II) fibres reinnervate spindles, fibres which fail to re-establish appropriate receptor innervation also fail to recover normal conduction velocity.(ABSTRACT TRUNCATED AT 400 WORDS)