Groups of 8-week old Crl: CD (tradename) male rats were exposed, head-only, to atmospheres of o-toluidine for single 4-hours periods. The LC50(sup double star) was determined to be 862 ppm with 95% confidence limits between 816 and 913 ppm. Compound-related clinical signs observed during and immediately following exposure consisted of tremors, slight to moderate cyanosis, muscle spasms, labored breathing, slight to moderate corneal opacity, prostration and semi-prostration, and reddish-brown nasal discharge. Clinical signs observed at 24 hours or longer consisted of corneal opacity, prostration, lethargy, labored breathing, pallor, hypothermia, reddish-brown nasal discharge, and stained-wet perinea. Weight losses of 6 to 22% were observed 1 to 3 days post-exposure, with normal weight gains occurring thereafter. The clinical manifestations of o-toluidine were generally dose-related. Deaths occurred with 1 to 4 days post-exposure. Time to death generally decreased as exposure concentrations increased.