The essential amino acid tryptophan is the precursor in the anabolic pathway of melatonin, a hormone with immunomodulatory properties. The present study shows the in vivo effect of tryptophan on the phagocytic function and oxidative metabolism of heterophils from Streptopelia roseogrisea of < 1 year of age, with a parallel evaluation of the plasma levels of melatonin. The L-tryptophan was administered orally (125 and 300 mg/kg b.w.) at 19:00, before the beginning of the period of darkness, for 7 days. At the end of the tryptophan treatment, determinations were made at 21:00 and 02:00 of the Phagocytosis Index, the Phagocytosis Percentage, the Phagocytic Efficiency and the superoxide anion levels in heterophils isolated from blood and of the plasma levels of melatonin. The results showed, for the determinations at 21:00 in the animals that had received 125 mg L-tryptophan/kg b.w., enhanced heterophil phagocytic function and raised levels of plasma melatonin, with no affect on the oxidative metabolism of the phagocytes. For the administration of the greater concentration of tryptophan (300 mg/kg b.w.), there were raised plasma melatonin levels together with increases in heterophil phagocytic capacity and phagocyte oxidative metabolism at 02:00. The results indicate that tryptophan administered orally at night to diurnal animals of less than 1 year in age affects the circulating levels of melatonin at the same time as inducing stimulation of the innate immune function.