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Age-related changes in the response of GH to administration of thyrotrophin releasing hormone (TRH) have been investigated in the domestic fowl. In two strains of chicken the i.v. administration of TRH (10 µg/kg) to 4-week-old male and female birds markedly increased (> 200 ng/ml) the plasma GH concentration within 10 min of treatment and the concentration remained higher than the pretreatment level for at least a further 20 min. Saline (0·9%) administration had no effect on GH secretion in comparable groups of control birds. The same dose of TRH had no effect on plasma GH concentrations in adult (> 24-week-old) laying hens or cockerels. The administration of TRH at doses of 0·1–100 µg/kg (i.v.) or 0·39–50 µg/bird (s.c.) also had very little, if any, effect on GH secretion in laying hens. In laying hens slight increases (10–20 ng/ml, P < 0·05) in the plasma concentrations of GH were observed in one experiment 60 min after the s.c. injection of 100 µg TRH, and in another 60, 90 and 120 min after the serial s.c. injection of TRH (100 µg/bird) every 30 min over a 150 min period. The poor GH response of the adults to TRH stimulation was not due to high circulating concentrations of endogenous gonadal steroids, as surgical gonadectomy had no effect on the GH response to TRH. These results suggest maturational differences in the control of GH secretion in the fowl.