>The Japanese government requires risk assessment of chemicals under the Chemical Substances Control Law (CSCL). Toxicity data for polyoxymethylene (paraformaldehyde; CAS No.: 30525-89-4) for human health are insufficient though the chemical needs a screening assessment under the CSCL. Thus, polyoxymethylene was selected by the Safety Examination of Existing Chemicals and Safety Programmes of the Ministry of Health, Labour and Welfare (MHLW) to assess repeated-dose and reproductive/developmental toxicity. A combined toxicity screening was conducted following the OECD TG422. Male and female rats were administered the test chemical once daily by gavage at doses of 0 (control), 20, 60, or 200 mg/kg bw from 14 days before mating for a total of 28 to 61 days. The 200 mg/kg bw/day dose caused a significant decrease in food consumption. Histopathological examination found ulcers in the forestomach and glandular stomach, and erosion and inflammatory cell infiltration in the submucosa of the glandular stomach at the end of dosing in both sexes. Inflammatory cell infiltration in the submucosa of the glandular stomach was also observed in both sexes after the recovery period. No reproductive and developmental toxicity was observed even at the highest dose. A no-observed-adverse-effect level (NOAEL) for repeated-dose toxicity was 60 mg/kg bw/day, and a NOAEL for reproductive and developmental toxicity was 200 mg/kg bw/day, the highest dose tested.