Cutaneous pectoris muscles of frog (Rana temporaria) were investigated 19.5-40 months after denervation. On whole mounts a heavy reduction in size and number of muscle fibres is noticed; in two muscles studied with semithin and ultrathin sections the number of remaining muscle fibres is 149 and around 120, while one of the contralateral muscles contains 250 and control muscles of equal sized frogs between 220 and 320 (n = 18) fibres. By electron microscopy muscle fibres undergoing degeneration or phagocytosis can be seen (3 of 20 muscle fibres present in a single ultrathin cross-section). On the other hand several profiles contained within one common basal lamina sheath are present in 14 of 20 fibres, indicating satellite cell proliferation. In one preparation 40 months after denervation not a single muscle fibre or axon is present, suggesting that eventually, without nerve supply, muscle fibres entirely disappear. Upon spontaneous reinnervation or implantation of the hypoglossal nerve 16 months after denervation, synapses are formed with the remaining muscle fibres. When studied 3.5-24 months after nerve implantation muscles innervated by few axons only (less than 10, 10-20 axons) contain a low number of muscle fibres (mean 44 +/- 41 SD, n = 6), while all muscles with a larger number of axons have more than 150 muscle fibres (n = 6). This indicates that unless large numbers of axons regenerate and/or when reinnervation is delayed muscle fibre loss continues to occur. The presence in one muscle of motor axons but only six muscle fibres 24 months after nerve implantation indicates that muscle fibre loss cannot be reversed, or recovery is extremely slow. This observation is interpreted as evidence for the exhaustibility of the satellite cell pool.