Relationships between serum thyroid stimulating hormone (TSH) levels, carcinogen dose and development of thyroid tumors were studied in Wistar rats of both sexes treated with N-bis-(2-hydroxypropyl)nitrosamine (DHPN). DHPN (210 mg/100 g body wt) was injected i.p. weekly. Wistar rats were used, divided into five groups of males (M1-M5) and five groups of females (F1-F5). M1 and F1 received one injection of DHPN, M2 and F2 two injections of DHPN, M3 and F3 three injections of DHPN, M4 and F4 four injections, and M5 and F5 as control rats received saline vehicle. The resultant incidences of total thyroid tumors were 4% in group M1, 24% in M2, 80% in M3, 76% in M4, 0% in F1, 4% in F2, 20% in F3, 17% in F4 and 0% in M5 and F5. Thus dose dependence for DHPN carcinogenesis was found in both sexes for up to at least three injections of DHPN and in all cases males proved more susceptible than females. Similar results were gained for carcinomas treated separately. A sex difference in DHPN induction of thyroid tumor was thus shown for Wistar rats opposite to that reported to exist in humans. However, no clear relationship between dose of DHPN or incidence of thyroid tumors and serum TSH concentration was evident at the end of the experiment (week 30).