The influence of estradiol treatment on the urinary excretion of relaxin, a hormone in earlier years only found during pregnancy and presently associated with functions in the cardiovascular system, was investigated in postmenopausal women. Thirteen postmenopausal women were treated with transdermal estradiol and 12 women with oral estradiol for 4 weeks. A new radioimmunoassay for human-relaxin (rec-hRLX-2) was used. With transdermal, but not with oral administration, a significant increase of urinary relaxin excretion was registered. Further experiments are necessary to elucidate the source of urinary relaxin and its role in the hormone replacement therapy of postmenopausal women.