An antiserum cross-reactive against ovine neurophysins-I-II and -III has been used in conjunction with the immunoperoxidase histochemical procedure to localize the cells of the sheep paraventricular (PVN) and supraoptic nuclei (SON). In order to describe the topographical distribution of the SON and PVN a study was made on the serial sections cut (a) transversely from rostral to caudal positions and (b) sagittally from lateral to medial positions of the hypothalamus. The cells of the SON, when examined in the transverse aspect, extended approximately 1900 mu caudally and when examined in the sagittal plane were contained within a lateral-medial distance of 4830 mu. In each case the SON cells lay adjacent to the optic chiasm. As sections were cut transversely, the cells of the PVN first appeared in a rostral position defined as 0mu and close to the ventral lining of the third ventricle. This general ventral and ventro-lateral distribution of cells maintained up to a caudal distance of approximately 840 mu. From positions 1260-2310mu there was a dramatic dorsal shift of the PVN cells which by this time had also extended laterally. The total rostral-caudal distance occupied by the PVN cells was 3150mu. That the lateral-medial distance occupied by the PVN was small (1050mu) was determined on examining the magnocellular nuclei in sagittal section.