Summary Dimethyl sulfoxide and water were compared as solvents for determination of LD50 values by the oral route in rats and mice using 10 quaternary ammonium salts as test compounds. Toxicity changes were obtained in some instances with 50% DMSO; more changes were observed in rats than in mice and the two species did not always parallel each other. When toxicity was altered by DMSO; it increased in all instances except one. Slope was changed in only 2 tests, one each in rats and mice but not with the same compound.