Luteinizing hormone releasing hormone (LHRH) and somatostatin content of the two halves of the median eminence of the rat hypothalamus were determined by radioimmunoassay three weeks after following deafferentations: (1) unilateral knife cut in front of, laterally and above the preoptic area (preoptic deafferentation); (2) unilateral arched cut at the midlevel of the suprachiasmatic region (suprachiasmatic deafferentation); (3) unilateral arched cut behind the optic chiasm (retrochiasmatic deafferentation); (4) cutting around the medial basal hypothalamus on one side starting with the cut at the posterior level of the optic chiasm (complete deafferentation). Preoptic deafferentation caused a more than 50% decrease in the LHRH content of the ipsilateral half of the median eminence. The reduction in LHRH was even greater (about 70%) in rats with suprachiasmatic deafferentation and only about 15% of the neurohormone was found in the ipsilateral half of the median eminence after unilateral retrochiasmatic or complete deafferentation. The median eminence somatostatin showed practically no change after unilateral preoptic deafferentation, while it was reduced by about 50, 60 and 80% in the ipsilateral half median eminence after a unilateral suprachiasmatic, retrochiasmatic and complete deafferentation, respectively. The results are consistent with the assumption that the majority of the LHRH perikarya projecting to the median eminence are located partly in the preoptic-anterior hypothalamic area and partly in brain regions in front of or above this area. Most of the somatostatin perikarya with terminals in the median eminence are in the preoptic-anterior hypothalamic region.