The investigations performed revealed that sulfhydryl compounds (reduced glutathione, cysteine and mesna) reduced the acute and subacute toxicity of 5-fluorouracil in mice. The above compounds changed pharmacokinetics of 5-fluorouracil increasing its accumulation in tissue compartment. The studies on 5-Fu distribution revealed its increased concentration in certain organs after administration of only certain sulfhydryl-containing compounds and only after certain doses.