The redox level of [¹⁴C] coenzyme Q₁₀ solubilized with HCO-60 in the livers of guinea pigs at 24 h after intravenous injection was approximately 1/4 (18.2% vs. 63.4%) of that of [¹⁴C] coenzyme Q₁₀ solubilized with ethanol-water (1 : 5 by vol). Further, the redox level of coenzyme Q₁₀ (Q₁₀) solubilized with HCO-60 or ethanol-water was 20.2 or 82.3%, respectively, after a 30-min incubation in the liver cytosol. Q₁₀ solubilized with HCO-60 was thus reduced only slightly in vivo or in vitro. The critical micelle concentrations of HCO-60 in the absence and presence of Q₁₀ were 0.02 and 0.002% (w/v), respectively. The concentrations of HCO-60 micelles containing Q₁₀ were estimated to be 0.0048% (w/v) in the incubation mixture with cytosol and 0.0041% (w/v) in blood circulation. These results suggest that HCO-60 micelles containing Q₁₀ would remain more stable not only in blood circulation but also in tissues as compared with ethanol-water emulsion. A possible mode of solubilization of Q₁₀ is discussed.