Methyl methacrylate (MMA) was administered by the inhalation route for a single 2-hour exposure and for 5 hours a day for 5 successive days at nominal exposure levels of 100, 400, 700 and 1000ppm to groups of male Alderley Park, Wistar-derived rats. A negative (air) and a positive (benzene 7500ppm) control group were also included. MMA causes weak, but statistically significant, increases in chromosome damage in rat bone marrow cells at some exposure levels and at the sampling times investigated by comparison with the negative control values both after single and multiple exposures. There is not, however, a consistent dose response and the effects are of an indeterminate toxicological significance in view of the overall negative toxicological profile of this compound.