Pituitary Content and Plasma Levels of Growth Hormone in Foetal and Weanling Rats

SUMMARY Pituitary and plasma growth hormone were measured in foetal and weanling rats. Growth hormone was first detected in the pituitary (66 ± 8 ng) and plasma (26 ± 2 ng/ml) of 19·5-day-old foetuses. Pituitary growth hormone content increased rapidly up to the end of gestation and after birth. Plasma levels were high after 21·5 days of gestation (147 ± 8 ng/ml) and a further increase was observed at birth. The plasma growth hormone level fell to that of adult levels 5–10 days after birth. An early modulation of growth hormone secretion is discussed since newborn rats aged 2 h showed a rapid decrease in plasma levels at 25 °C while there was no such decrease in newborn animals maintained at 37 °C.