Radiolabelled agonist and antagonist binding to porcine anterior pituitary dopaminergic receptor indicates the existence of two affinity forms of the receptor. These two forms of the receptor are modulated by guanine nucleotides. The agonist high affinity form of the receptor is converted to a low affinity form, whereas the antagonist low affinity form is converted to the high affinity form under the influence of guanine nucleotides. Thus, guanine nucleotides appear to reciprocally modulate the same two forms of the receptor discriminated by both agonists and antagonists. These observations may provide insights into the biochemical mechanism by which dopamine exerts its regulation of prolactin release in the anterior pituitary.