The subthalamic nucleus (ST) of kittens was studied by means of the rapid Golgi silver impregnation method. The neurons of the ST were classified into three types. Type I neurons, the main constituents of the ST, had oval or polygonal cell bodies with a mean diameter of 26 micrometer by 36 micrometer and four to six primary dendrites. Type II neurons had multipolar or polygonal cell bodies, which measured an average 31 micrometer by 43 micrometer and emitted four to seven primary dendrites. The cell bodies of the type III neurons were polygonal in shape, measured 23 micrometer by 26 micrometer in average and emitted four to six primary dendrites. The dendrite bundle and the dendrite pallisade were observed. Frequently dendrites of the ST extended into the cerebral peduncle (CP), and even cell bodies of some ST neurons were located within the CP. All of the parent axons of the ST neurons coursed rostrally, although intra- and extra-nuclear axon-collaterals arising from the ST neurons travelled rostrally, caudally or caudomedially. The afferent fibers to the ST were divided into three groups; afferents via the Meynert's commissure (MC), decending and ascending afferents. The MC fibers, which ran across the CP, gave terminals to the ST. The descending afferents were axon-collaterals of fibers descending in the CP and those of fibers running through the ST. The ascending afferents were also axon-collaterals arising from ascending fibers in the CP. In addition to these afferents, many descending and ascending fibers of passage ran through the ST without emitting axon-collaterals.