SUMMARY Growth hormone (GH) concentrations in the plasma of foetal lambs (range 20–150 ng/ml) were ten or more times those in the plasma of ewes during the last third of pregnancy. Blood samples obtained from foetuses with indwelling vascular catheters indicated that there was a steady rise in foetal plasma GH concentrations from 40–50 ng/ml at 100–110 days gestation to maximum values of 110–120 ng/ml between 130 and 140 days gestation and then a rapid decrease around the time of birth. Growth hormone disappeared from the foetal plasma after hypophysectomy of the foetus and there was no evidence for a placental source of the hormone. Ovine pituitary growth hormone given by intravenous injection disappeared from the foetal plasma at a similar rate to the endogenous hormone following hypophysectomy. The rate of GH removal from foetal plasma was slower than that in lambs 4–12 days after birth. Glucose infusion into foetal lambs did not alter their plasma GH concentration significantly, but infusion of the catecholamine, isoprenaline, decreased the foetal plasma GH concentration rapidly. Cessation of infusion was followed by very rapid restoration of the former high plasma GH concentration. The results indicate that the high foetal plasma GH concentration is maintained by active secretion of the hormone, but its function in the foetal lamb remains obscure.