The conditions necessary for retrograde transport of blood-borne peroxidase having been established, those required for orthograde transport within intact, undisturbed neurons are being investigated. Neuronal cell bodies and dendrites are exposed to horseradish peroxidase (HRP) by perfusing it through the cerebral ventricles. Neurons of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) are labeled by this route. A very small amount of HRP moves down the axons of these neurosecretory cells to accumulate with a few lysosomes in their axonal terminals located in the posteior lobe of the pituitary gland. The ingestion of a 1.5% NaCl solution, known to enhance the antergrade low of neurosecretory peptides (Gainer et al., 1977), also increased the amount of HRP that flows down the neurosecretory axons to the posterior lobe. However, the HRP remains within the cerebral extracellular fluid for long periods in the dehydrated mice. Thus, the increased amount of HRP in their axons may simply be due to a decreased turnover of CSF. Neuronal somata and dendrites are consequently bathed by HRP for a sufficiently long period for more protein to be incorporated and transferred to their axons.