Methylhydrazine, asym-dimethylhydrazine, and sym-dimethylhydrazine vapors are similar to hydrazine in that they are respiratory irritants an;d convulsants on acute exposure. In addition to causing respiratory distress and convulsions, methylhydrazine vapor was found to produce hemolysis and elevated temperatures in exposed dogs. Methylhydrazine was the most toxic of the vapors tested, and hydrazine the least toxic. The higher volatility of asym-dimethylhydrazine indicates that the hazard of this vapor is similar to methylhydrazine. However, the hemolytic properties of methylhydrazine indicate that this vapor has a greater sublethal hazard. Preliminary data on sym-dimethylhydrazine suggest that this vapor has a toxicity and vapor hazard that is less than methylhydrazine and asym-dimethylhydrazine but greater than hydrazine. The odors of hydrazine, methylhydrazine, and asym-dimethylhydrazine were usually described by human subjects as ammoniacal, fishy, or amine-like. The odor of these vapors appears to offer adequate warning of acute exposure to dangerous aspirations. 16 references, 2 figures, 7 tables.