The amount of cAMP in the pubic symphyses of estrogen-primed mice increases after injection of the hormone relaxin. This relaxin-induced increase in symphyseal cAMP was observed in immature, mature, and ovariectomized mature mice and could be prevented by prior ip injection of rabbit antibodies to porcine relaxin or by treatment of relaxin with dithiothreitol. The highest level of cAMP was measured 30 min after relaxin injection; the level of measurable cMAP then diminished rapidly. Estrogen-priming of the mice was not a prerequisite for a relaxin-induced response to occur. Relaxin administration did not increase the level of cAMP in a non-target tissue such as liver, nor could an increase in cAMP in the pubic symphysis be elicited by injection of insulin, a protein of similar size and structure, or glucagon, a known stimulator of liver cAMP levels.