The chemotherapeutic agent cis-platinum has been used effectively as a treatment for many tumors. One of the most common toxic effects caused by this treatment is ototoxicity. More severe hearing loss has been reported in patients who had received cranial radiation prior to chemotherapy with cis-platinum. The present laboratory study was undertaken with chinchillas to determine whether ototoxicity is potentiated by cis-platinum administration after cochlear irradiation. Results support the clinical observation that cranial radiation prior to administration of an ototoxic chemotherapeutic agent can have a synergistic effect of increased susceptibility to the ototoxic potential of the agent.