The pineal gland and the immune system in mammals and birds are functionally connected, with some recent findings showing that melatonin, the major secretory product of the pineal gland in vertebrates, is a free-radical scavenger and general antioxidant. In birds, heterophils, with their ability to ingest and kill different antigens, play a central role in the host defense mechanism. This in vitro study investigated the possible antioxidant action of melatonin in heterophils isolated from adult male and female ring dove (Streptopelia risoria). The superoxide anion levels (O2-.) and the superoxide dismutase (SOD) activity were evaluated as indicators of the metabolic burst of heterophils after ingestion of inert particles (latex beads) at the basal time (0 min) and 15, 30, 45, and 60 min of incubation with 100 microM melatonin. The heterophil O2-. levels, measured by the nitroblue tetrazolium reduction test, were lower after incubation in the presence of melatonin at all the times studied, a time-dependent effect. Latex beads produced a significant increase in SOD activity, whereas there was a significant decrease in the presence of melatonin. In addition, when heterophils were incubated in the presence of both latex beads and melatonin, the indole appeared to annul the enhancement of SOD activity (U-525nm) produced by latex beads. In conclusion, our findings show that the neurohormone melatonin could be a useful antioxidant.