The LD50's of various solvents frequently used in toxicological work to improve the solubility of otherwise poorly soluble compounds were determined in the mouse and rat. The substances investigated were ethanol, dimethylacetamide, dimethylformamide, dimethylsulfoxide, glycerine, N-methylpyrrolidone, polyethylene glycol 400, 1,2-propanediol and Tween 20. The substances were administered under standardized conditions to groups of 10 animals (5 females, 5 males) (mice and rats) at doses that supplied at least 3 values lying between the LD16 and LD84. The median lethal dose and slope were determined with the aid of Finney's programmed probit analysis. The results are tabulated as LD50 ml/kg with 95% confidence limits. It is recommended that when using these solvents for pharmacological and toxicological investigations not more than a quarter of the LD50 should be used because otherwise there will be confusion between the deaths due to the substance being investigated and those due to the solvent.