This study examined effects of exogenous melatonin on osmoregulatory hormones and water and sodium secretion by salt glands and excretion via the kidneys of Glaucous-winged gulls (Larus glaucescens). Six saline acclimated gulls were injected with inulin and paraminohippuric acid and then infused with 500 mM NaCl to stimulate salt gland secretion. Each bird was given infusions of NaCl alone and NaCl plus melatonin. Experiments were made one week apart in a randomized order. A large blood sample (to measure osmoregulatory hormones) was taken before infusion, at secretion, and at the end of infusion. A small blood sample was taken at the midpoint of each of six 10 min sequential collections of salt gland secretion and urine. Melatonin tended to increase plasma sodium concentration, did decrease plasma osmolality, but did not affect potassium concentration. Melatonin did not affect salt gland secretion rate or concentration nor renal plasma flow or glomerular filtration. Melatonin increased urine flow rate, tended to increase urine sodium concentration, and did decrease urine potassium concentration. Combined renal and extrarenal sodium excretion was greater during MT treatment. During NaCl infusion, angiotensin II increased, aldosterone decreased, and arginine vasotocin remained unchanged. Melatonin did not affect these responses. These data suggest an osmoregulatory role for melatonin in birds with salt glands.