Adult rat muscle fibres were dissociated by using collagenase and maintained in culture. One to nine days later, neurons obtained from stages 22-30 Xenopus laevis embryos, or neonatal spinal cord, or pheochromocytoma (PC12) cells treated with nerve growth factor were added. Subsequently, the co-cultures were maintained for up to eight days. Functional synapses were formed with variable efficiency: 12% in rat-Xenopus nerve-muscle co-cultures, 23% in rat-rat and 33% in PC12 co-cultures. Miniature endplate potentials (MEPPs) and currents (MEPCs) were recorded, at frequencies ranging from 0.01 to 0.9 Hz. Their mean amplitude was smaller than in normal mammalian muscles. The rise time and time-constant of decay of MEPCs was about seven to ten times longer than that found in the original muscle, resembling immature synapses. (+)-Tubocurarine abolished the MEPPs in the rat-PC12 neuromuscular junctions. It is concluded that dissociated adult rat muscle fibres retain their ability of being reinnervated, and can form functional synapses with foreign neurons and transformed chromaffin cells.