Variations to immunoreactive growth hormone (GH) and bioassayable somatomedin-like activity (SLA) were determined in the plasma of the domestic fowl. The plasma concentrations of SLA (potency relative to a pooled serum standard) and GH were both inversely age-related in growing cockerels (Thornber 909) between 3 and 20 weeks of age (SLA, r = 0.454; P less than 0.01, GH, r = 0.548; P less than 0.01) and directly related to each other (r = 0.54; P less than 0.01). In immature birds maintained on a photoperiod of 16 h light, 8 h darkness, plasma concentrations of SLA were highest during the first hour of darkness (1.19 +/- 0.18 units/ ml (u/ml) mean +/- S.E.M., n = 6) and significantly lower 1 h after dawn (0.78 +/- 0.07 u/ml n = 6). No diurnal variations in plasma concentrations of GH were observed. Deprivation of food in two strains of chickens (Thornber 909 and broiler) significantly reduced plasma concentrations of SLA in both strains and increased plasma concentrations of GH in the Thornber strain. Re-feeding fasted Thornber chickens caused the plasma concentrations of SLA and GH to return to normal within 24 h but had no effect in broilers. Deprivation of water, which also reduces food intake, for 12, 18 and 24 h markedly reduced plasma concentrations of SLA but had no effect on plasma GH concentration in Thornber cockerels.