Testosterone exerts a negative feedback action on the secretion of gonadotrophin releasing hormone (GnRH) and the gonadotrophins in males. The mechanism by which this occurs is unclear, as GnRH neurons do not express androgen receptors or oestrogen receptor-α. One possibility is that the feedback effect is transmitted through GnIH cells. GnIH is a peptide produced in the dorsomedial nucleus (DMH) and GnIH neurons project to GnRH cells and to the median eminence in the sheep brain. GnIH is secreted into the hypophysial portal blood in this species and acts to inhibit GnRH action on gonadotropes. To test the hypothesis that testicular hormones regulate GnIH gene expression, we used in situ hybridisation histochemistry to measure levels of mRNA in the hypothalamus of rams and wethers (n=5/group).  Using 20µm cryostat sections of perfusion-fixed ram tissue 5 regions of the hypothalamus were examined. The number of GnIH cells detected was higher (P<0.05) in rams (55.5±12.21) than in wethers (11.5±7.54) in the caudal region of the DMH. Expression/cell was measured counting silver grains with the ³⁵S-labelled riboprobe, but this was similar in rams and wethers. We conclude that testicular hormones increase GnIH mRNA expression in the ram hypothalamus and that this contributes to the negative feedback effect on GnRH and gonadotropin secretion. Future work will determine whether this feedback effect is due to testosterone.