The neuronal population of the spinal cord lamina I (marginal zone) was structurally characterized, in the cat, by the use of the Golgi method complemented by multivariate analysis of morphometric data. Four cell types were identified, two of them including two subtypes. Fusiform cells accounted for 43% of impregnated cells and presented flame-shaped rostrocaudally elongated perikarya and bipolar, either strictly longitudinal (fusiform A; 37%) or longitudinal and ventral (fusiform B; 6%) dendritic arbors with numerous short-pedicled spines. Fusiform cells preferentially occupied the lateral one-third of lamina I. Multipolar cells (22%) had ovoid perikarya with bulging surfaces and numerous primary dendritic trunks. Two subtypes could be distinguished: multipolar A cells (12%) with highly ramified dendrites covered with variably shaped spines and multipolar B cells (10%) with looser and less spiny dendritic arbors expanded for longer distances. Multipolar cells were more commonly found in the medial half of lamina I. Flattened cells (16%) possessed discoid perikarya flattened across the dorsoventral axis and aspiny, scarcely ramified dendritic arbors distributed horizontally within lamina I. They predominated in the intermediate one-third of the lamina. Pyramidal cells had triangular prismatic perikarya partially encased in the white matter overlying lamina I. They represented 19% of the impregnated neurons and were located along the entire lateromedial extent of the lamina. Each neuronal type included a few cells with perikarya and dendritic arbors three times larger than the rest. These so-called giant cells amounted to 6% of the entire lamina I neuronal population. According to the present data, the neuronal population of the spinal cord lamina I of the cat strongly resembles that of the rat (Lima and Coimbra,