1. Frog muscles are isolated and maintained in organ culture conditions for periods of up to 2 months. During the first 2 weeks, muscle fibres have normal resting membrane and action potentials. Subsequently the potentials decline in amplitude.2. Slow muscle fibres also survive in culture and retain their ability to give maintained contractures.3. Muscle sensory receptors continue to function in culture until the axon terminals degenerate at about 2 weeks.4. Neuromuscular transmission is normal during the first few days of culture, after which the motor endings degenerate. Transmission persists longer (up to 17 days) if a long segment of nerve is left attached to the muscle. With short-nerve preparations failure of transmission in vivo occurs at about the same time as in culture. With long-nerve preparations failure of transmission is delayed even further in culture.5. In short-nerve preparations miniature end-plate potentials disappear, in general, at about the time that transmission fails. In long-nerve preparations some end-plates continue to have miniature end-plate potential activity for a short time after nerve impulses cease to evoke any response; but eventually miniature potential activity disappears from all end-plates.6. After a few days of electrical silence, miniature end-plate potentials reappear at some of the denervated end-plates. The proportion of denervated end-plates which show miniature end-plate potentials in culture is smaller than in muscles denervated in situ.7. Electron microscopy shows that muscle structure is well preserved in culture, that the axons degenerate and that the Schwann cells move to occupy the space vacated by the axons. The Schwann cells are very probably the source of the acetylcholine which evokes miniature potentials in the denervated end-plates.