Male and female Fischer 344 rats and B6C3F1 mice were exposed to 0, 5, 25 or 75 ppm acrylic acid vapors 6 hours per day, 5 days per week, for 13 weeks. These exposure levels were selected after conducting a 2-week probe study in which 225 ppm caused pronounced growth retardation and nasal lesions in both rats and mice. The 13-week exposures had no adverse effect on the growth of male and female rats and male mice. However, mean body weight gains of female mice in the 25 and 75 ppm exposure groups were statistically significantly lower than for controls after 12 weeks of exposure. There were no pronounced treatment related effects on organ weights, hematologic parameters, clinical chemistry parameters or urinary parameters. Histopathologic examinations revealed lesions of the nasal mucosa in rats in the 75 ppm exposure group, and in some or all mice at each treatment level. The nasal lesions were primarily localized to the olfactory epithelium; the respiratory epithelium was relatively unaffected. The histopathologic observations in both rats and mice included degeneration, and inflammatory cell infiltration in the olfactory mucosa. In mice there were also instances of hyperplasia of the submucosal glands and, replacement of olfactory epithelium by respiratory epithelium. These effects were attributed to the irritant properties of acrylic acid vapors.