The existence of melatonin receptors in adrenal glands is suggested by effects on rodent adrenal function which follow pinealectomy or in vivo and in vitro melatonin treatment. In 1992, Persengiev and coworkers reported specific binding of 2-[125I]iodomelatonin to rat adrenal tissue. In order to study adrenal binding further we have done binding studies on adrenal membranes from ducks sacrificed at midlight. We observed binding of 2-[125I]iodomelatonin which was specific, rapid, saturable, stable, reversible and of high affinity. Scatchard plots were linear and Hill coefficients were close to unity supporting the existence of a single class of 2-[125I]iodomelatonin binding sites. On Scatchard analysis the Kd was in the physiological range (27.4 pmol/l) together with a Bmax of 3.38 fmol/mg protein. Sites were highly specific to melatonin and its two synthetic analogs, 2-iodomelatonin and 6-chloromelatonin in pharmacological studies. In autoradiographic studies on the chicken adrenal gland, one class of 2-[125I]iodomelatonin binding site was demonstrated with a Kd of 58.8 pmol/l and Bmax of 182 fmol/g tissue. Kinetic and pharmacological studies indicated that these sites are saturable, reversible, and of high specificity and affinity. These findings in the chicken are similar to those of the duck adrenal data obtained by radioreceptor assay. In preliminary studies, no significant difference was found between 2-[125I]iodomelatonin binding to the adrenal glands collected at midlight and middark. These high-affinity binding sites are similar to those reported in a wide variety of tissues and are consistent with the hypothesis of direct action of melatonin on the adrenal gland.(ABSTRACT TRUNCATED AT 250 WORDS)