Sprague-Dawley rats were exposed to acrylonitrile by inhalation at 40, 20, 10, 5 and 0 ppm, 4 hours daily, 5 days weekly, for 52 weeks and at 60 ppm, 4-7 hours daily, 5 days weekly. The latter treatment was started on 13-week-old breeders, and male and female offspring (12-day-embryos). The breeders and part of the offspring were exposed for 104 weeks; the other part of the offspring was exposed for 15 weeks only. Sprague-Dawley rats were also exposed to acrylonitrile by ingestion (stomach tube), in olive oil, at 5 mg/kg b.w., once daily 3 times weekly for 52 weeks. Under the tested experimental conditions, acrylonitrile was shown to be carcinogenic in Sprague-Dawley rats when given by inhalation and did not produce any carcinogenic effect when given by ingestion. In the inhalation experiment, acrylonitrile caused an increase in different types of tumors and the most noticeable acrylonitrile-related tumor was encephalic glioma.