Coenzyme Q10 (CoQ10) is an essential cofactor in the mitochondrial electron transport pathway, and is also the only known endogenously synthesized lipid-soluble antioxidant. The aim of the present study was to determine, for the first time, endogenous plasma CoQ10 concentration and its correlation with plasma total antioxidant capacity (TAC) and serum total cholesterol (TC) in a population of healthy untrained horses. Thirty-one horses were included in the study. Plasma CoQ10 concentration ranged from 0.380 to 2.090 mg/L, which is in general agreement with plasma CoQ10 concentration in humans. The study demonstrated no significant correlation between plasma CoQ10 and TAC, which indicates that CoQ10 does not contribute to the TAC of equine plasma significantly. In contrast to humans, no significant correlation was found between CoQ10 and TC in investigated horses. The results warrant further studies on CoQ10 supplementation in healthy untrained horses and subsequent determination of correlations between CoQ10 and TAC.