The post-natal development of pyramidal tract neurones (p.t.n.s) was investigated in twenty-one barbiturate-anaesthetized kittens from birth to 28 days of age using a combination of electrophysiological and anatomical techniques. P.t.n. responses were recorded intracellularly as well as extracellularly with glass micropipettes filled with horseradish peroxidase (HRP) on stimulation of the medullary pyramid and cerebellar nuclei. Latency histograms of antidromic responses of p.t.n.s were compared at various ages. In the neonate, p.t.n.s were divided into two groups which were presumed to be analogous with fast and slow p.t.n.s in adult animals. During the first post-natal week, latency shortening was not conspicuous, but by the end of the second post-natal week, the faster group showed a marked decrease of latencies (up to around 10 ms at 14 days of age), while those of the slower group did not change so much. The slower group increased their conduction velocity during the third post-natal week (latencies up to around 18 ms). At the end of the fourth post-natal week, the distribution of antidromic latencies was in a narrower range, but the values were still longer than those reported in adult animals. Intracellular HRP staining revealed that apical dendrites of p.t.n.s spread fully to the pial surface even at birth. The somata of these neurones were characteristically covered with somatic appendages and development of the basal dendritic tree was immature in 0-1-day-old kittens. Basal dendrites developed nearly completely by 7 days, but somata were still covered with appendages. During the fourth post-natal week, these appendages disappeared almost completely. The sizes of the dendritic field, especially of apical dendrites, became larger in parallel with the development of cortical layers. From the morphological point of view, differentiation of fast and slow p.t.n.s was not clear until 28 days of age except in somatic volumes, which were already different in the first post-natal week. At the end of the fourth post-natal week, p.t.n.s with short antidromic latencies had a tendency to bear spines more sparsely over the secondary and tertiary dendritic surface in comparison with p.t.n.s with longer latencies. Intracortical axonal trajectories developed fairly well in the immature cerebral cortex, and the general pattern of ramification changed little during the first month after birth.(ABSTRACT TRUNCATED AT 400 WORDS)