The topological positions of nerve terminal sprouts in partially denervated rat sternocostal muscles were analysed. Sprouted nerve terminals were found only within 200 microns of a denervated muscle fibre, but were not necessarily within 200 microns of a denervated endplate. From this it was concluded that the sprout-inducing factor released from denervated muscle fibres does not necessarily arise from the denervated endplate, but may be released along the length of the muscle fibre. It is proposed that sprout factor is released during the incorporation of acetylcholine receptors into the muscle fibre membrane.