The sub-acute inhalation toxicity of furfural was studied in Syrian golden hamsters. Four groups of ten males and ten females each were repeatedly exposed to furfural vapour at concentrations of 0, 20, 115 and 552 ppm (0, 77, 448 and 2165 mg/m3 air) respectively for 6 hours/day, 5 days/week during a period of 13 weeks. At the highest exposure level furfural induced irritation of the eyes and nose, slight growth retardation and atrophy along with hyperplasia of the olfactory epithelium in the nasal cavity. At 115 ppm atrophy and hyperplasia of the olfactory epithelium were the only adverse effects observed that could be attributed to furfural. No compound-related alterations were detected at the lowest exposure concentration of 20 ppm (77 mg/m3 air) which was, therefore, accepted as a no-toxic effect level.