Changes in 2[125I]iodomelatonin binding to the chicken spleen following disruption of the normal melatonin rhythm by a slow-release melatonin implant were investigated. Melatonin capsules were implanted in 6-week-old male and female chickens reared under 12 h light/12 h darkness. The mid-light and mid-dark serum melatonin was measured throughout the period of implantation. Spleen weight and characteristics of splenic 2[125I]iodomelatonin binding sites were studied during the mid-light at 4 or 8 weeks after implantation. Melatonin implantation disrupted the normal diurnal rhythm of serum melatonin and elevated the mid-light level to the control mid-dark value until 5 weeks after implantation. The melatonin implant caused a significant decrease in the density of 2[125I]iodomelatonin binding sites at 4 weeks after implantation while there were no significant changes in the binding affinity. The reduction in 2[125I]iodomelatonin binding may be a result of down-regulation of the melatonin receptors by elevated serum melatonin. Alternatively, it may be due to an indirect suppression of 2[125I]iodomelatonin binding sites by the elevated sex hormones since melatonin implants increased the levels of testosterone in the male and estradiol in the female. Implantation of melatonin capsules significantly increased the body and spleen weights in the male chicken. These weight changes may be partly explained by the anabolic and erythropoietic action of the increased circulating testosterone.