Alaptide is a drug chemically derived from prolyl-leucyl-glycin amide (PLG) which is effective after oral administration. We studied the effect of long term treatment with alaptide and estradiol-benzoate on serum prolactin, growth reactivity and dopamine DA-2 receptors in the anterior pituitary of male rats.Alaptide reduced adenohypophyseal weight when given alone, but only nonsignificantly reduced growth reactivity of the anterior pituitary (AP) raised by estradiol-benzoate (EB).Alaptide significantly decreased the serum level of prolactin but, on the other hand, significantly increased the binding of3H-spiperone to dopamine DA-2 receptors in AP membrane preparations, without affecting affinity.The administration of alaptide plus EB together resulted in an additional increase of dopamine DA-2 binding sites.We assume that alaptide has a weak dopaminergic activity on the AP of male rats.