The origins of substance P immunoreactive axons in the small intestine of the guinea-pig were investigated with an immunohistochemical technique in whole mount preparations. Nerve pathways were interrupted either in vitro or in vivo to detect the accumulation of substance P proximal to the lesion and the disappearance of immunoreactive fibres resulting from the degeneration of the severed axons. Various operations, namely, extrinsic denervation, interruption of the myenteric plexus (myotomy) or removal of the myenteric plexus with the longitudinal muscle (myectomy), were performed prior to examination of substance P-containing neurons. There are several projections of substance P-containing neurons which supply the intestine. Extrinsic neurons are the sources of two projections, one to submucosal blood vessels and one to the submucous ganglia. Intrinsic neurons located in the submucous ganglia supply the villi. Five projections arise from the myenteric plexus, a very short projection ending either within the same row of ganglia or within the adjacent rows of ganglia on both sides, a longer projection within the myenteric plexus, a very short projection to the circular muscle, a projection to the submucous ganglia where the axons surround most of submucous nerve cell bodies, and a projection to the villi. It is likely that the highly organised patterns of innervation by different substance P-containing neurons have specific roles in the intestine. Some of these neurons may act as sensory neurons, others as interneurons, and yet others as motor neurons in nerve pathways within the enteric nervous system.