Hydrocyanic acid (CAS No. 74-90-8) was evaluated for subacute inhalation toxicity in Sprague-Dawley rats (5/sex/group) administered whole-body exposures to a mean analytical concentration of 67.8 +/- 7.0 ppm for 6 hours on 3 consecutive days under dynamic conditions (2060-2200 L/min). An initial exposure was associated with hypoactivity, rapid shallow breathing, anoxia/hypoxia signs, and convulsions, followed by chromorhinorrhea, convulsions and varied signs of labored breathing. Survivors of 2 exposures exhibited rapid breathing and reduced mean bodyweight with a solitary male rat exhibiting arching of the back. Signs of toxicity associated with a third exposure were limited to hypoactivity and quick shallow breathing; mean daily bodyweight gains were restored. Mortality occurred either during or following an initial exposure in 1 and 2 males, respectively. Necropsy of these study decedents revealed cyanosis of the extremities, hemorrhagic lungs, lung and tracheal edema, blanching of the liver, chromorhinorrhea, urine-filled bladder, gaseous distention of the gastro-intestinal tract, and a solitary incidence of blood engorgement of the heart and surrounding vasculature. On terminal necropsy of study survivors, females exhibited slight to moderate hemorrhage (3/5) and grey discoloration (2/5) of the lung.