2-Isocyanatoethyl ester methacrylic acid (CAS No. 30674-80-7) was evaluated for acute inhalation toxicity. The test substance was evaluated in Fisher 344 rats (10/sex/group) at concentrations of 2, 4, and 7 ppm for six hours, followed by fourteen days of observation. The test atmosphere was generated by the use of a Rochester-type inhalation chamber. Mortality was not observed during exposure. After exposure, mortality was observed at 4 ppm (6/10 M, 3/10 F) and 7 ppm (7/10 M, 7/10 F). The LC50 for males was 4.44 ppm and 5.27 ppm for females. Clinical observations at 2 ppm were slight. At 4 and 7 ppm all rats showed progressively increased signs of eye and nasal irritation and respiratory difficulties during and after exposure. Most rats that died within the 48 hour post-exposure period had accumulations of porphyrin-containing secretions on the facial region (eyes, nose, and/or mouth). At all levels of exposure, animals showed initial weight loss. Gross pathologic examination revealed no grossly visible lesions in any of the male rats exposed to 2 ppm, but revealed focal pneumonia in the lungs of 5/10 and focal atelectasis in the lungs of 1/10 female rats. Congestion of the lungs, rhinitis, patchy areas of pneumonia, inflammatory exudate in the trachea and/or bronchi, and increased quantities of gaseous material and decreased ingesta within the gastrointestinal tract were revealed from examination of internal organs in rats exposed to 4 and 7 ppm. In a second study, 2-isocyanatoethyl ester methacrylic acid (CAS No. 30674-80-7) was evaluated for acute inhalation toxicity. The test substance was evaluated in six one-hour exposures to Fisher 344 rats (1/sex/group) at concentrations of 10, 20, or 40 ppm followed by fourteen days of observation. The test atmosphere was generated by the use of a small 150-liter Rochester-type inhalation chamber. No mortality was observed during exposure. The LC50 for males was 2.42 ppm and 36 ppm for females. Clinical observations included initial weight loss at all concentrations. All rats showed signs of eye and nasal irritation and respiratory difficulties from exposure to 20 and 40 ppm. Gross necropsy revealed no lesions to rats exposed to 10 ppm. Rats exposed to 20 and 40 ppm revealed focal cloudiness of the cornea, lesions of the respiratory system, congestion of the nasal mucosa, focal pneumonia, failure of lungs to collapse normally upon opening the trachea and bronchi, exudative material around external nares, congestion of the nasal turbinate, and upper respiratory obstruction.