SUMMARY Since blood oestrogen levels are much higher during pregnancy in the goat than in the sheep a continuous isotope infusion technique was used to study oestrone metabolism in five experiments on two goats during late pregnancy and early lactation. The metabolic clearance rate was similar to that in the sheep in all experiments (mean value ± s.e.m., 4·02 ± 0·25 1/min); therefore the high blood oestrone concentrations in late pregnancy (851–2043 pg/ml) were due to high oestrone production rates (4·22–7·54 μg/min). There was some conversion of oestrone to oestradiol-17β by the mammary glands and other tissues (mean conversion ratio 17·1%) and a little (< 10%) binding of oestrone to erythrocytes. The uptake of oestrone by the mammary gland corresponded to < 3% of the total production rate. These results are compared with those obtained for other steroid hormones (progesterone, cortisol) in the goat and sheep.