Ribavirin, a synthetic nucleoside with marked antiviral activity, induced developmental malformations when administered to pregnant hamsters by oral, intraperitoneal or intravenous routes. Abnormalities of the limbs, eyes and brain were the most common defects found in the hamster. Higher doses (about 10 x) were required to induce anomalies in rat embryos and the malformations were generally restricted to the head region. In both rats and hamsters oral administration of the drug seemed to be more teratogenic than administration by other routes suggesting that metabolism of ribavirin in the maternal gastro-intestinal tract and/or liver may change it into its active form.