Exposure to nonsteroidal antiinflammatory drugs (NSAIDs) during pregnancy has been reported to adversly affect both human and animal embryonic development. Since diclofenac sodium is one of the most prescribed NSAID, we examined its effect on pregnant rats and their offspring. In the present study, animals were treated with 1 mg/kg i.m. diclofenac sodium beginning on the 5th day of gestation until the 19th day. In the same period control pregnant rats were given 1 ml physiological saline. After 28 days of birth the offspring were fixed by perfusing 10% buffered-formalin. Paraffin sections of the cerebellum were stained with luxol fast blue and neutral red. The cerebellar cortical layer thicknesses and Purkinje cell density were estimated. No teratogenic effects were found in the diclofenac-treated rats. However, the Purkinje cell density but not cortical layer thicknesses of the diclofenac-treated rats were significantly reduced when compared with their controls. These results do not support the suggestion that diclofenac sodium is teratogenic, but they do suggest that daily doses of 1 mg/kg diclofenac sodium interferes with development of the cerebellum.