Physiological and behavioral (conditioned avoidance) responses of male Long-Evans rats were determined during exposure to combustion products produced on thermal degradation of three different polymeric materials. Arterial blood samples were obtained for determination of carboxyhemoglobin (COHb) and acid/base status. Material A produced a syndrome of carbon monoxide (CO)-induced anoxia, the severity of which was a function of the mass of material degraded. Material B produced grand mal seizures despite COHb levels of less than 10%. Material C produced metabolic acidosis and a mild degree of CO-induced anoxia. Loss of avoidance responses occurred at significantly lower COHb levels for materials B and C in comparison to CO alone. Using responses to COHb as a reference, it was possible to detect the presence of other toxicants present in combustion products. Compounds found in smoke in very low concentrations may have a high degree of biological activity and be responsible for impairment of survival responses. We have labeled these compounds "limiting" toxicants. They constitute a significant hazard, which is added to that of CO and anoxia.