2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a toxic agent and has disruptive effects on reproductive tissues in females. TCDD disrupts the hormonal regulation of the body and decreases the production of melatonin. In this study, we investigated the protective effects of melatonin supplements against the toxic effects of TCDD on ovaries of female rats. TCDD caused a significant decrease in the average number of corpora lutea and follicles per tissue section (2.1 +/- 0.7; 2.3 +/- 0.8, respectively), whereas these numbers were maintained in the melatonin supplemented group (5.0 +/- 0.8; 5.1 +/- 0.8, respectively) and were similar to the control group (5.3 +/- 1.0; 5.9 +/- 0.9, respectively). Electron microscopic analysis showed that the disruption of ultrastructure components such as cell membrane and organelles due to TCDD exposure was inhibited by melatonin supplements. This study suggested that melatonin has a protective and a possible ameliorative effect over histopathological damage of rat ovaries exposed to TCDD.