On the basis of dendroarchitecture and cell body shape, complemented with morphometry of dendritic ramification, four major neuronal types were distinguished in lamina I of the spinal cord of the rat. (I) Fusiform spiny cells (39% of impregnated neurons) have longitudinal spindle-shaped perikarya with bipolar, less frequently unipolar, dendritic trees rich in pedicled spines and a thin, beaded longitudinal axon; such neurons occur mainly in the lateral marginal zone. In type IA cells (33% of the total), the dendritic domain occupies a narrow longitudinal area, while in type Ib cells (6%) the polar dendritic trees partly arborize ventrally. Fusiform neurons are considered intrinsic cells connected with the longitudinal afferent plexus in lateral lamina I, and in type IB cells also receiving primary input in the substantia gelatinosa. (II) Multipolar cells (23%) have a dense dendritic arbor originating from numerous primary trunks and they predominate in the medial marginal zone. The dendritic arbor is moderately extended dorsoventrally in type IIA cells and reaches lamina III in the larger type IIB cells. The former possess a variety of spines, axonlike processes and sometimes an unmyelinated axon, and are presumably interneurons, while type IIB cells show a thick tapering axon that is probably myelinated. (III) Flattened aspiny neurons (13%) with a polygonal body flattened in the horizontal plane, and a horizontal dendritic arbor confined to lamina I; these cells predominate in middle lamina I. (IV) Pyramidal neurons (25%) have longitudinally elongated perikarya that bulge into the white matter. The arbor has a large longitudinal and lateromedial spread and includes branches which ramify in the white matter. Types III and IV show the classical lateromedially elongated orientation of the marginal cells of the old literature; they show thick tapering axons and probably make up the bulk of the projection neurons of rat lamina I.