A monocomponent, high specific activity, carrier-free porcine relaxin tracer (125I) has made it possible for us to demonstrate relaxin receptors in the symphysis pubis, uterus, and ovary via autoradiography. The receptors are concentrated in the symphyseal ligament and the peripheral layers of uterus and ovary. Specific relaxin binding was observed in crude membrane preparations of uteri, ovaries, and brain, whereas crude membranes of leg muscle and kidney showed only nonspecific binding. Uterine membranes prepared from estrogen-primed mice showed tracer binding, which could be significantly inhibited by porcine relaxin in a dose-dependent manner, but not by insulin. A linear Scatchard plot suggested the presence of only one kind of receptor and a dissociation constant of 5 x 10(-10) M, which is commensurate with an electrostatic double ion pair binding mechanism.