The carcinogenicity of eleven alpha-oxidized nitrosamines (alpha-acyloxy, alpha-hydroperoxy and alpha-oxo nitrosamines) was tested in F-344 rats. All chemicals were dissolved in olive oil and rats received 10 weekly subcutaneous injections of these chemicals (10 x 5 mg/kg of N-methyl-N-(acetoxymethyl) nitrosamine or equimolar amounts of the other chemicals) at the interscapular region. Subcutaneous tumours were detected in many rats of all groups treated with the chemicals, although no tumour was detected at the injection site in the control group. Lung and thyroid tumours were also observed in many rats of the treated groups, especially those injected with N-alkyl-N-(acetoxymethyl)nitrosamines. Many other tumours developed in the experimental groups, but it is not certain that they were related to the treatment with the chemicals. The results indicate that the chemicals possess local as well as systemic carcinogenicity in F-344 rats. The potent carcinogenic effects at the injection site of the alpha-oxidized nitrosamines, coupled with their direct mutagenic activity, suggest that these derivatives are useful models for the ultimate form of the metabolically activated nitrosamines.