The organotropic effect of orally administered N-bis(2-hydroxypropyl)nitrosamine (DHPN) in male F344 rats was studied with respect to total dosage and length of observation period. DHPN was given at 5 dosages, a single intragastric intubation of 800 mg/kg body weight, or administration as a 0.2% solution in the drinking water for 4, 7, 14 or 21 days. Animals were killed at weeks 32, 40 and 48. Tumors were induced in the lung, thyroid, kidney and urinary bladder. High tumor incidence was demonstrated in the lung and thyroid. Dose-related increases in overall tumor incidences in these 4 organs were clearly shown but time-related increases at the 3 sacrifice time points were not clear. Histologically, the tumors were adenoma and adenocarcinoma of the lung and thyroid, renal cell tumor, nephroblastoma and transitional cell tumor of the kidney, and transitional cell papilloma of the urinary bladder.