Recent investigations have suggested that endogenous dopamine may have a physiologic role as a circulating or local hormone, as well as a neurotransmitter. The results of this study indicate that prolactin, and possibly LH secretion may be altered by acute changes in Hypophyseal-portal vein DA, within the physiologic range. It is unlikely that circulating DA modulates secretion of aldosterone, PRA, or other pituitary hormones in normal subjects. DA concentrations of 4 ng/ml, and possibly much higher, would be needed at local renal sites to affect sodium balance.