The subchronic inhalation toxicity of ammonium persulfate was characterized using Sprague-Dawley rats (20/sex/group) at respirable dust concentrations of 0, 5.0, 10.3, and 25 mg/m(3). Whole-body exposures were conducted 6 h/day, 5 days/wk for 13 wk. Gravimetric airborne test material samples were taken daily and particle size samples were taken weekly from each exposure chamber for analysis. Ten animals/sex/group were necropsied after 13 wk of exposure, and 5 animals/sex/group were held for 6- and 13-wk recovery periods. Animals were observed for clinical signs. Effects on body weight, food consumption, clinical chemistry and hematology, ophthalmologic parameters, organ weights, gross lesions, and histopathology were evaluated. There were no exposure-related deaths during the study. Rales and increased respiration rate were noted in both males and females in the 25 mg/m(3) group, and in a few animals in the 10.3 mg/m(3) group. The incidence of these clinical signs decreased to zero during the first few weeks of the recovery period. Body weights for both males and females in the 25 mg/m(3) group were significantly depressed during most of the exposure period compared to the control group. By the end of the recovery period, body weights for the exposed animals were similar to the control group values. Lung weights were elevated in the 25 mg/m(3) group after 13 wk of exposure, but were similar to controls at 6 wk postexposure. Irritation of the trachea and bronchi/bronchiole was noted microscopically after 13 wk of exposure to 25 mg/m(3). These lesions had recovered by 6 wk postexposure. Based on the results of this study, the no-observed-adverse-effect level (NOAEL) was 10.3 mg/m(3), while the no-observed-effect level (NOEL) for exposure of rats to a dust aerosol of ammonium persulfate was 5.0 mg/m(3).