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1. Plasma growth hormone (GH) and prolactin concentrations were measured between 1 and 24 weeks of age in both sexes of a laying strain of chickens and from 1 to 9 weeks of age in broiler cockerels.
2. GH and prolactin concentrations were highest in young birds and higher in males than in females.
3. In both sexes GH and prolactin concentrations were correlated inversely with age and body weight and positively with relative weight gain.
4. Differences in growth rate between broilers and layers were related to differences in blood GH concentrations.
5. Water deprivation for periods of 6, 12, 18 or 24 h increased plasma prolactin markedly but did not affect GH secretion significantly.