Rats bearing a transplanted pituitary tumor which secreted prolactin and growth hormone (GH) were injected with L-dopa. A significant fall in serum prolactin occurred by 30 min; by four hr the serum prolactin values returned to baseline concentrations. Adrenergic blockade induced by propranolol and phentolamine had no independent effect on serum prolactin or GH concentrations. Phentolamine, however, in hibited and propranolol prolonged L-dopa suppression of serum prolactin. An ip injection of L-dopa acutely suppressed serum GH by 60 min only after the animals had been pre-treated with L-dopa. It is suggested that L-dopa suppressed serum prolactin through direct inhibition of tumor prolactin secretion via the a-adrenergic system. (Endocrinology91: 1314, 1972)