Experiments were conducted to study the effect of goldthioglucose (GTG) upon the processes associated with lipid peroxidation. The glucose-6-phosphate dehydrogenase activity (G6PD; E.C. 1. 1.1.49) in red blood cells (RBC) and the amount of malonaldehyde precursors (MDA) per gram of brain, liver and kidney were determined. Adult mice received i.p. injections for three consecutive days of either saline (controls) or GTG dissolved in saline, in a dose of 0.10 mg.g(-1) or 0. 15 mg.g(-1) b.w. In mice receiving higher dose of GTG the G6PD activity was significantly increased (349.38+/-17.46 mU.10(-9) RBC compared to 258.2+/-14.46 mU.10(-9) RBC in control animals). The content of MDA precursors rose significantly from 4.8+/-0.81 micromol.g(-1) of the liver in controls to 8.12+/-1.41 micromol.g(-1) and 7.88+/-0.51 micromol.g(-1) and from 18.71+/-1.01 micromol.g(-1) of the kidneys in controls to 24.25+/-1.25 micromol.g(-1) and 24.88+/-1.7 micromol.g(-1) respectively. The GTG-induced higher levels of MDA precursors and increased G6PD activity in RBC corresponds to the rise in lipid peroxidation and its participation in producing the lesions after experimental and therapeutic use of gold-containing substances seems possible.