Uterotrophic response in sexually immature female rats has been used to rank the relative estrogenic potencies of six resorcylic acid lactones (RALs) and to compare their activities with that of 17ß‐estradiol. On oral administration, the estrogenic potency relative to 17ß‐estradiol is as follows: 7α‐zearalenol, 10 times less; zeranol, 150 times less; taleranol, 350 times less; zearalanone, 400 times less; zearalenone, 650 times less; 7ß‐zearalenol, 3500 times less. On subcutaneous administration, zeranol is 500 times less estrogenic than 17ß‐estradiol.