It has been reported that coenzyme Qsub(10) (CoQsub(10)) functions as an electron transfer carrier in mitochondria, and can produce an improvement in heart diseases such as congestive heart failure. Its (2Z)-isomer contains a cis-double bond at the 2-position of the decaprenyl side chain. As the original organic industrial synthesis of CoQsub(10) resulted in a product that contained a small amount of this isomer, the efficacy and safety of CoQsub(10) was determined using CoQsub(10) containing this isomer; however, no toxicity data have been reported for the (2Z)-isomer itself. Thus, we conducted single (2,000 mg/kg) and 4-wk repeated (1,000 mg/kg) oral dose toxicity studies in rats to compare the toxicological profiles of CoQsub(10) and its (2Z)-isomer. The two compounds displayed similar toxicological profiles, and it was concluded that neither CoQsub(10) nor its (2Z)-isomer produce toxic effects in rats in single or repeated doses