Horseradish peroxidase (HRP), injected into the rat caudal medulla oblongata, was detected by immunoperoxidase staining in 120 microns frozen sections, allowing examination of both the distribution and morphology of transporting neurons. In the hypothalamus, several groups of HRP-labeled neurons could be distinguished on the basis of location of the neurons, neural cell size and morphology of the neural processes. Most HRP-labeled neurons were found in the posterior half of the hypothalamus, although scattered single neurons were present as far rostral as the anterior hypothalamus and preoptic area. Prominent groups of HRP-labeled neurons were found in the paraventricular, dorsomedial and arcuate nuclei, near the fornix at two separate levels, and in the lateral posterior hypothalamus. Based on comparison with peptide immunohistochemistry it seems likely that many magnocellular oxytocin, vasopressin and neurophysin neurons in the paraventricular nucleus, and a few ACTH/beta-endorphin neurons in the arcuate nucleus may project to the caudal medulla oblongata.