Nitropyrenes are a class of polycyclic hydrocarbons that have been shown to be potent point mutagens in the Salmonella/mammalian microsome assay. However, relatively few data have been published on their ability to induce chromosome aberrations. In this paper we report the cytogenetic activity of 1-nitropyrene, 1-nitrosopyrene, and 1-aminopyrene in cultured Chinese hamster cells (Don:Wg3h). Following treatment with 1-nitrosopyrene, high levels of both chromosome and chromatid types of aberrations were induced, the most notable effect being an increase in chromatid exchanges. Much weaker, but statistically significant, responses were also obtained for 1-nitropyrene and 1-aminopyrene.