Immobilization stress had a biphasic effect on serum prolactin levels: the early short stimulatory phase followed by a long inhibitory phase in male rats. Stress induced the rise in serum prolactin without concomitant increase in serum TSH levels which declined during the immobilization for 300 min. Other stressors, ether inhalation or formalin s.c. injection, or TSH i.v. injection, which were effective in controls failed to elevate serum prolactin after the 300-min immobilization. Serum TSH responded to TRH after the stress as well. Pimozide, dopamine receptor blocker, was effective in increase of serum prolactin in the stressed rats as well as in controls. In pimozide pretreated rats, elevated serum prolactin levels decrease in 10 min by the immobilization and returned to the preimmobilization levels thereafter which were higher than those in stressed animals without pimozide treatment. It is suggested that TRH is not a physiological PRF in the stress-induced prolactin release and that the dopaminergic system may be activated by the immobilization stress, resulting in decrease of the prolactin-releasing activity of the pituitary.