SUMMARY A continuous isotope infusion technique was used to measure the metabolic clearance rate (MCRB) and production rate (PRB) of oestradiol-17β in a group of six non-pregnant conscious Chinchilla rabbits. The mean MCRB of oestradiol was 162 ml/min (0·079 1/ min/weight0·75), and at equilibrium a significant proportion of the radioactivity in the circulation was present as oestrone (conversion ratio oestradiol to oestrone, 27·6%). The mean PRB of oestradiol was 4·3 ng/min, which is equivalent to about 5 μg/day. Some day-to-day variation was seen in the endogenous concentrations of oestradiol, oestrone and progesterone although in general the levels of the two oestrogens were less than 30–40 pg/ml, and progesterone levels were below 300 pg/ml apart from short peaks up to 600 pg/ml. These values were compared with previous measurements of the ovarian secretion rate of oestradiol measured directly in anaesthetized animals.