Reinnervation of extensor digitorum longus muscle following crush of sciatic nerve was studied in rats made hypothyroid after weaning. In vitro intracellular recordings of muscle cell postsynaptic potentials were carried out; moreover twitch and tetanus following direct muscle stimulation and nerve stimulation were recorded. Frequency of miniature end-plate potentials (mepps) may be regarded as an index of presynaptic mechanism in regenerated nerve endings: when regenerating axons reach the muscle, the frequency of spontaneous acetylcholine quantal release is very low and increases in subsequent weeks. No significant differences were noted in miniature end-plate potential frequency between muscles of normal and hypothyroid rats at the same time from denervation; mepp amplitude was higher in hypothyroids, in accordance with the smaller muscle fibre diameters. Regenerating nerve fibres entering the muscle extensively sprout, giving rise to a number of nerve endings which exceeding the number of muscle cells, are subsequently withdrawn; correspondingly, muscle cells are transiently polyinnervated and the number of polyinnervated muscle cells peaks decreases subsequently approximating zero. The percentage of polyinnervated cells peaked sooner in hypothyroid rats than in controls and afterwards decreased; a tail of polyinnervation was found at long term. Tension recording experiments showed a shorter time of reinnervation of muscles in hypothyroid rats, but no difference in regeneration rate could be argued. These findings suggest an influence of thyroid hormones in the stabilization of motor innervation of reinnervated muscle, but not in nerve regeneration process.