cAMP content, phosphodiesterase activator and protein kinase activity can be increased in anterior pituitary of rat following intraperitoneal injection of reserpine or isoniazid. The effects of reserpine can be blocked by pargyline, (a monoamine oxidase inhibitor). The effect of isoniazid is not abolished by pargyline but is abolished by small doses of muscimol, diazepam and nipecotic acid, all drugs that increase the GABAergic function. Muscimol, diazepam and nipecotic acid are ineffective against reserpine. These experiments suggest that noradrenaline and GABA exerts a independent inhibitory role on the function of the hypothalamic-pituitary system.