This Golgi study identifies three neuronal cell types in the substantia gelatinosa (SG) layer of the spinal trigeminal nucleus. The SG neurons are distinguished from each other based on: (1) dendritic branching pattern, (2) denritic spine distribution, (3) geometric shape of the denritic tree, (4) laminar distribution of the dendrites, (5) axonal branching pattern and (6) laminar distribution of the axonal arbor. The islet cell is found in small clusters and its dendrites and axonal arbor are confined within the SG layer. Its dendrites span the full width of the SG layer and extend up to 500 mum in the long axis of the layer. Dendritic spines are generally sparse with small clusters of spines found on the higher order dendritic branches. The islet cell axon extends for at least 1 mm in the long axis of the layer. Each of its collaterals divide every 50-100 mum with one branch doubling back in the direction of the cell body and the other branch continuing on in the direction of its parent. In this manner each islet cell generates a profuse axonal plexus in the SG layer. The stalked cell is found individually within the SG layer. Its cell body is usually found in the inner half of the SG layer and its sinuous dendrites cross the SG layer and enter the marginal layer. The stalked cell dendrites emit numerous fine stalk-like branches and dentritic spines. Its axon emits branches in the SG and marginal layers. The spiny cell is found singly between groups of islet cells. Its extensive dendritic tree spans up to 500 mum rostrocaudally and mediolaterally crossing into both the marginal and magnocellular layers. Spiny cells have evenly distributed dendritic spines along their dendrites in the SG layer. The spiny cell axon sends branches into all three layers of nucleus caudalis. Numerous branches enter the outer 300 mum of the magnocellular layer where they undergo further branching with some branches returning in recurrent fashion toward the SG layer. The three neuronal cell types of the SG layer satisfy all of the morphological criteria for Golgi type II interneurons. Their highly branched axons generate many collaterals within the confines of their dendritic trees and do not project out of nucleus caudalis. The SG neurons are considered to be inhibitory interneurons interposed between V nerve primary afferent axons which arborize in the SG layer and second order neurons of nucleus caudalis.