Allylisothiocyanate (AITC) has been evaluated for its ability to initiate unscheduled DNA synthesis (UDS) in the livers of male rats in vivo. Specific Pathogen Free outbred albino Hsd/Ola Sprague-Dawley rats were exposed by oral gavage to 37.5 or 125 mg/kg AITC in corn oil and hepatocytes assessed for UDS by autoradiography 2 and 14 h later. AITC did not induce UDS at either dose level at either time point. These data are consistent with all other evidence indicating that AITC does not act as a genotoxin in vivo, despite positive response in some in vitro screening assays. The reported occurrence of benign bladder papillomas in male rats but not female rats or mice of either sex is consistent with non-genotoxic action and may be attributed to chronic irritation of the bladder epithelium by AITC and its cysteine conjugate metabolite excreted by male rats in unusually concentrated form. It is concluded that the weight of evidence is insufficient to regard AITC as a genotoxin capable of human carcinogenicity.