Whole-of-life tests of the C-nitroso compounds p-nitrosodimethylaniline (NDMA) and p-nitroso-diethylaniline (NDEA) have been completed in male rats and mice fed maximum tolerated doses continuously over the first halves of their respective natural lifespans. The chemicals were offered at a concentration of 300 mg/litre drinking fluid, but the doses of NDEA consumed were only 75% of the NDMA doses, in both species. Possibly because of this the results with NDEA were statistically not clear-cut, but there was a significant increase in tumour incidence after NDMA treatment in both species. The main sites of tumorigenesis after NDMA were lung, kidney and malignant lymphoma in the rats, and lung, duodenum and malignant lymphoma in the mice. The results confirm our own earlier experiment and provide the first evidence of oncogenic activity in this class of compounds.