Serum relaxin levels were analysed in 12 healthy women every other day during the menstrual cycle and during a second cycle on oral contraceptives. Relaxin levels in 7 women with posterior pelvic and lumbar pain were also measured. Relaxin was detected during both the follicular and luteal phases of the menstrual cycle in some of the healthy women. Serum levels were further increased during the use of oral contraceptives. Oestradiol levels in the untreated women correlated to the relaxin levels. Women with posterior pelvic and lumbar pain had higher relaxin levels than did healthy women, a finding that needs to be further explored. Our data indicate the existence of sources for relaxin production other than the corpus luteum in the non-pregnant woman. Endogenous and exogenous oestrogens may stimulate the production of relaxin.