The effect of exogenous melatonin on embryonic and post-embryonic development of quail was analysed. Japanese quail (Coturnix coturnix japonica) eggs were incubated under standard conditions and light regime L12:D12. At incubation day 5 eggs were treated with 5, 50 or 250 m g of melatonin per egg and at incubation day 10 with 50 or 250 m g melatonin/egg. Melatonin was injected into eggs in 50 m l of sterile saline containing maximum 5% of ethanol. Fertility and hatchability of eggs were recorded. Hatchlings were inspected for gross developmental aberrations and killed by decapitation during post-embryonic day 1, 2 and 3 in the middle of the scotophase and photophase. Plasma and pineal glands were collected and melatonin levels were measured in both materials by radioimmunoassay. Weights of the liver, spleen and bursa of Fabricius were recorded at postembryonic day 1, 2 and 3. Plasma melatonin levels were increased in melatonin treated groups in comparison with control group at day 1 and 2. High hormone levels persisted until post-hatching day 2. These findings suggest that melatonin was not metabolized efficiently during the embryonic period and embryos were under melatonin influence during embryonic development. Administration of melatonin did not affect pineal melatonin levels. No developmental malformations were found in treated birds. Hatchability and weights of organs did not differ between the melatonin -treated and control groups. Melatonin, quail, development, embryoMelatonin, produced by the pineal gland, is involved in the control of circadian rhythms.