Catecholamine (CA) content of the ventral hypothalamus of the domestic fowl was measured under conditions and treatments that would provide markedly different levels of gonadotrophins. Hens were examined before and after a release of gonadotrophin in the ovulatory cycle. The hypothalami of cocks with variously sized testes and combs, as well as the hypothalami of castrates, were analyzed. The CA concentration in the hypothalami of both sexes was measured after adenohypophysectomy. Increased hypothalamic CA occurred concomitant with increased gonadotrophin activity. The CA content of the White Leghorn hypothalamus was found to be 3–16 µg/g wet weight of tissue, the amount depending largely upon circulating gonadotrophin levels. The hypothalamic catecholaminergic system is probably involved in regulation of releasing factors in the chicken. The ‘short-feedback’ mechanism does not function in this system. Testicular function and events in the ovulatory cycle and clutch sequence may be regulated by it.