The role of the subfornical organ in the timing of birth in the rat was investigated. Animals with radiofrequency lesions of the subfornical organ made on day 12 of pregnancy gave birth significantly earlier (P < 0.05) than intact and control-lesioned rats. Animals with lesions made on day 19 of pregnancy gave birth within control times. In addition, the natural fall in plasma relaxin observed at the end of gestation in rats was prevented by i.v. infusion of porcine relaxin (4.2 micrograms/h for 5 days from day 19 of gestation), which maintained plasma relaxin levels at approximately 100 ng/ml. This rate of infusion was selected because the resultant circulating levels of relaxin reflect plasma concentrations observed on day 20 of pregnancy in rats. The effect of lesion of the subfornical organ was then studied on the timing of birth in relaxin-infused rats. Intact animals and rats with control lesions receiving an infusion of relaxin had significantly (P < 0.05) prolonged pregnancies compared with intact saline-infused controls. However, the timing of birth of rats with lesions of the subfornical organ receiving an infusion of relaxin was not significantly (P > 0.05) different from that in intact saline-infused controls. The results support the hypothesis that the subfornical organ appears to mediate the central effects of relaxin and may have a natural role in the events that lead to delivery in the rat.