Immunoreactivity for gamma-aminobutyric acid is located in one morphologically-defined class of nerve cell body in the myenteric plexus of the guinea pig small intestine. These are a subgroup of the Dogiel type I nerve cells, characterized by their lamellar dendrites, about 1 micron thick and flattened in the plane of the myenteric plexus, and one (or rarely two) long axonal process that extends to either the longitudinal or the circular muscle. At an ultrastructural level the dendrites were characterized by their open cytoplasm in which were scattered granular vesicles, pale mitochondria, Golgi apparatus and endoplasmic reticulum. A large proportion of the dendritic surface was in direct contact with the extra-ganglionic space. In the cell body region, which was away from the ganglion surface, the nucleus was surrounded by a thin rim of cytoplasm. The cytoplasmic features are quite distinct from those of Dogiel type II neurons but they were shared by many other non-immunoreactive neurons. Synaptic inputs, which were all non-immunoreactive, were found on the dendrites, cell bodies, axon hillocks and axons of the gamma-aminobutyric acid-immunoreactive neurons. The predominant vesicle type in the presynaptic elements was the small clear vesicle, 40-60 nm in diameter. Based on two gamma-aminobutyric acid-immunoreactive cells that were examined in serial section, about 40-50% of synapses are dendritic, 20-25% are somatic, and 30-35% are on the axon hillock or first 50-70 microns of the axon. No synapses formed by immunoreactive varicosities were found on non-immunoreactive neurons or in the neuropil of the myenteric ganglia. Moreover, the lamellar dendrites or soma of gamma-aminobutyric acid neurons were never presynaptic elements forming relationships with other elements in the ganglia. It is concluded that the gamma-aminobutyric acid reactive Dogiel type I neurons are motor neurons providing inputs to the circular and longitudinal muscle layers.