Immunohistochemical methods were used to determine the localisation of immunoreactivities to a variety of antigens involved in neurotransmission in the myenteric plexus of the colon in the rat and mouse. The findings in the two species were closely similar. Five neuronal types have been identified. (i) The axons of extrinsic noradrenergic sympathetic neurons, immunoreactive for tyrosine hydroxylase, supply the ganglia and the circular muscle. (ii) Bombesin immunoreactive intrinsic neurons with unbeaded axons are largely confined to the ganglia and tracts of the plexus. These neurons probably contain gastrin-releasing peptide, which is the mammalian analogue of bombesin. (iii) Somatostatin immunoreactive intrinsic neurons have long, beaded axons within the myenteric plexus and also outside the plexus, between the longitudinal and circular muscle layers. (iv) Intrinsic neurons containing opioid peptides (beta-endorphin, met-enkephalin, leu-enkephalin), have beaded axons that cannot be traced for long distances. They contact all the cell bodies in the ganglia and extend also into the interganglionic tracts and the smooth muscle. (v) Substance P immunoreactive somata and axons are present throughout the myenteric plexus and provide dense innervation to the smooth muscle. Extrinsic substance P immunoreactive sensory axons are probably also present.