A strategy has been developed to identify and quantify the different neurochemical populations of myenteric neurons in the guinea-pig ileum using double-labelling fluorescence immunohistochemistry of whole-mount preparations. First, six histochemical markers were used to identify exclusive, non-overlapping populations of nerve cell bodies. They included immunoreactivity for the calcium binding proteins calbindin and calretinin, the neuropeptides vasoactive intestinal polypeptide, substance P and somatostatin, and the amine, 5-hydroxytryptamine. The sizes of these populations of neurons were established directly or indirectly in double-labelling experiments using a marker for all nerve cell bodies. Each of these exclusive populations was further subdivided into classes by other markers, including immunoreactivity for enkephalins and neurofilament protein triplet. The size of each class was then established directly or by calculation. These distinct, neurochemically-identified classes were related to other published work on the histochemistry, electrophysiology and retrograde labelling of enteric neurons and to the simple Dogiel morphological classification. A classification scheme, consistent with previous studies, is proposed. It includes 14 distinct classes of myenteric neurons and accounts for nearly all neurons in the myenteric plexus of the guinea-pig ileum.