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Melatonin, a putative pineal hormone, has been proved to be active orally. In the present investigation the efficacy of melatonin as an oral antifertility agent in mice was investigated. Melatonin tablets prepared by the solvent deposition technique (SDT) were found to be superior in their physical properties and the release rate of melatonin from the tablets was higher than those prepared by the conventional mixing method (CMM). The interaction of melatonin with polyvinylpyrrolidone (PVP) and lactose may be responsible for its high release rate. Melatonin tablets prepared by the SDT were used to evaluate its antifertility effect in mice. This effect was compared with that of its intramuscular injection. Oral administration of melatonin on the proestrus day resulted in 100% reduction in the mean number of corpora lutea and lutein cells. The ovulation blockage ratio was 100%. The mean duration of the oestrus phase was significantly decreased. Absence of corpora lutea was evident histologically. On the 4th day of pregnancy, oral administration of melatonin led to 100% interruption of pregnancy and 100% reduction in the mean number of implantation sites. Also, the average number of the blood capillaries and their transverse diameter were significantly decreased. Histological examination of the interrupted pregnant uteri showed completely degenerated embryo. The result of oral administration of melatonin mimicked the known effects of its intramuscular injection.