N-Nitrosodibenzylamine (NDBzA) is mutagenic to Salmonella typhimurium and induces DNA strand breaks in isolated rat hepatocytes, yet it is reported to be non-carcinogenic to the rat. Here we report that it is inactive in both the rat and mouse bone marrow micronucleus assays and in a rat liver autoradiographic assay for unscheduled DNA synthesis. It is, however, clearly active as a micronucleus-inducing agent and mitogen in the rat liver and is capable of inducing single-strand breaks in the DNA of rat liver. The origin and implications of this curious conflict of in vivo genotoxicity data are discussed. Irrespective of that discussion, it is concluded that NDBzA is genotoxic to the rat liver in vivo.