We studied the oncogenic action of neonatal streptozotocin (STZ) treatment in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) for 12 months. Two-day-old male neonates were intraperitoneally injected with STZ of which doses were 37.5-75.0 mg/kg for SHR and 100.0-150.0 mg/kg for WKY. The 12-month survival rate was 16 of 22 (73%) in SHR and 10 of 14 (71%) in WKY, respectively. The incidence of tumors in STZ-treated SHR was 27% in liver, 14% in kidney and 5% in liver and kidney, being related to the dose of STZ given, namely, 25% in 37.5 mg/kg, 50% in 50.0 or 62.5 mg/kg and 75% in 75.0 mg/kg. In STZ-treated WKY which survived 12 months, all had tumors, namely, 70% in liver, 20% in kidney and 10% in liver and kidney. Histological features of liver and kidney tumors were characteristic of hepatoma and nephroblastoma, respectively. Islet cell tumor was evident in 4 of 10 (40%) in SHR treated with lower doses of STZ (less than or equal to 50 mg/kg) but not in SHR and WKY treated with higher doses (62.5-150.0 mg/kg). The present study indicates that neonatal STZ treatment has the oncogenic action on liver, kidney and pancreatic islet.