Fast blue, true blue and fluorogold injected into neonatal and subadult rats via subcutaneous or intraperitoneal route labelled certain forebrain nuclei and the central and peripheral neurons whose axons form the spinal and most cranial nerves. The A1 and A5 noradrenergic neurons, nucleus of the tractus solitarius and some neurons in the nucleus raphe medianus, nucleus reticularis gigantocellularis were also labelled. In addition, occasional fluorescent neurons were seen in the vestibular nucleus, locus coeruleus, fastigial nucleus and Purkinje cell layer of the cerebellum. Some nuclei labelled in neonatal rats hardly showed any labelled neurons in subadult rats. After injection the subarachnoid space, blood vessels in the spinal cord, brain and peripheral ganglia, the circumventricular organs and the choroid plexuses were labelled much faster than the neurons. A similar pattern of labelling was seen in subadult rats receiving intravenous injection of fluorogold. Evans blue and rhodamine injected intravenously failed to label any neurons in the brain or spinal cord.