The rate of removal of immunoreactive, intravenously injected 125I-labelled sheep growth hormone (GH) was used to calculate metabolic clearance rates (MCR) in two foetal lambs at 130 days of gestation and in two 6-day-old lambs. The mean MCR calculated for the foetuses was 2·9 ml/min/kg and for the lambs 3·1 ml/min/kg. The concentration of GH in plasma sampled before injection was determined immunologically and the values were used to calculate production rates. A production rate of 924 ng GH/min was calculated for the foetuses and 85 ng GH/min for the lambs. The effect of sectioning the pituitary stalk was studied in two foetuses; after the operation there was a rapid decrease in the circulating levels of GH. Hypophysectomy in two other foetuses also caused an abrupt decrease in plasma GH concentration.
It was concluded from these experiments that the exceptionally high concentrations of GH in the plasma of foetal lambs could not be attributed to impaired removal of the hormone from the circulation. The direct cause of the increased hormone concentrations was a high rate of GH secretion resulting from active stimulation of the foetal pituitary by the hypothalamus.
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