Since the first observations of HISAW, the knowledge on the chemistry and the biological properties of relaxin improved in an important way. It is now well known that relaxin is a peptide hormone, strictly related to insulin. The extraction and purification of porcine relaxin and the application of radio-immunological and immuno-histochemical methods have provided a lot of data concerning the activity and the sites of production of the hormone. Recently human relaxin was synthesized. Besides the well known effects on the pubic symphysis and the uterus further actions were detected in the ovary, in the mammary gland, in the seminal plasma etc... Relaxin is produced not only by the corpus luteum, as at first believed, but also by decidua, endometrial glands and, in the male, by the prostatic gland. Some findings suggest a possible production by follicular cells too. In fact, relaxin was detected in the follicular fluid and the production of the hormone by porcine granulosa cells was demonstrated in vitro. Our immuno-histochemical research has shown the presence of relaxin-like substances in the granulosa cells of cavitary follicles of the human and porcine ovary.