Porcine relaxin (250 guinea-pig units/mg) infused intravenously into anaesthetized rats at 20 μg/h reversibly abolished spontaneous intra-uterine pressure cycles yet left the myometrium responsive to oxytocin in doses of 4–8 mu. The inhibition was found to be primarily of the frequency, rather than of the amplitude, of pressure cycles. Relaxin (5 or 10 μg) was capable of completely suppressing uterine activity driven by prostaglandin F2α infusion in oestrogen-treated ovariectomized rats. Whereas the β-adrenergic blocker, propranolol, had no effect on relaxin-induced inhibition, phentolamine, an α-blocker, significantly delayed the relaxin effect. It is unlikely, however, that relaxin operates through an α-inhibitory receptor. The results show that relaxin acts primarily as a frequency modulator and is capable of antagonizing an exogenous myometrial stimulant.