The effect of ubiquinones Q-l, Q-2, Q-8 and Q-9 on lipid metabolism in rat thymocytes in vitro was studied. The cells were incubated in a medium containing ubiquinones within the concentration range from 1 to 100 #M. A 2-fold decreased cholesterol synthesis was observed in thymocytes incubated with ubiquinone Q-9 at a concentration of exogenous ubiquinone of no less than 40 #M. Incubation of thymocytes with ubiquinones UQ-I and UQ-2 that are characteristic of rats (40 #M and I00/tM) resulted in a decrease of cholesterol synthesis. Ubiquinone-8 had a tendency to inhibit the cholesterogenesis in rat thymocytes.