The effect of oral administration of diazoxide on rats bearing mammary carcinomas induced by dimethylbenzanthracene (7,12-DMBA) or methylnitrosourea (MNU) was investigated. Administration of 300 mg/kg diazoxide caused mild reversible diabetes with maximum glucose levels of 305 +/- 74 (control: 119 +/- 12) mg/dl and related insulin levels of 15 +/- 5 (control: 24 +/- 11) microU/ml after 4 hr in tumor-bearing animals. Following the same dose of diazoxide a more than 90% inhibition of tumor growth was observed in 7,12-DMBA- and MNU-induced autochthonous rat mammary carcinomas as well as remission of the median total tumor volume per group in 7,12-DMBA-induced lesions. Frequently, onset of remissions and median remission duration proved to be dose-dependent in 7,12-DMBA-induced mammary carcinoma and, with the exception of the median remission duration, in MNU-induced tumors too. After cessation of diazoxide application, 30% rebound responses were observed in 7,12-DMBA-induced tumors of animals that had had a first remission due to diazoxide. Application of insulin (2 IU per rat) together with diazoxide (300 mg/kg) reversed the tumor-inhibiting effect of diazoxide in MNU-induced tumors. The diazoxide effect might in part be due to a decrease in the percentage of proliferating cells caused by insulin depletion as indicated by a lower amount of cells in S-phase, as measured by DNA-flow cytometry. Marked toxicity was observed after effective doses of diazoxide; the experiments indicate that induction of reversible diabetes might be a useful tool in the treatment of hormone-dependent mammary carcinoma.