The present investigation was to determine whether there are any combined effects occurring in laboratory animals undergoing such exposures and to extrapolate for possible health hazards to the operator. The presence of cyanide vapor was not considered likely to increase the toxicity of maleic anhydride since the acute toxicity of maleic anhydride is primarily due to the irritation it causes in the respiratory tract. But since acrylonitrile is acutely toxic primarily from its cyanide action, it was considered possible that respiratory tract irritation which could result from maleic anhydride could increase the absorption of acrylonitrile vapors and thereby increase its effect. Exposure of laboratory animals to maleic anhydride vapors followed by acrylonitrile vapor or to both compounds simultaneously, did not reveal, under the experimental conditions, any significant difference in survival time. Exposures of animals to both compounds in sequence revealed that their effects were independent of each other. However, when the animals were subjected to both compounds simultaneously, additive responses were observed.