The liver fluke Opisthorchis viverrini infection is a major health problem in northeastern Thailand. We have previous indicated the short-term treatment with praziquantel induces inflammation and oxidative/nitrative stress. This study, we evaluated the effect of melatonin and optimized route of administration (oral and intraperitoneal (i.p.) injection) on inflammation and oxidative/nitrative stress induced by short-term treatment with praziquantel in O. viverrini-infected hamsters. The result showed that melatonin treatment dose-dependently decreased the inflammatory cells and the levels of oxidative/nitrative stress markers. In contrast, melatonin increased the level of antioxidant power. Oral administration had more effective on these parameters than the i.p. injection groups. Therefore, oral administration of melatonin may be used as a chemopreventive agent to reduce the severity of the disease during the short-term praziquantel treatment.