This study focused on the mitochondrial DNA (mtDNA) as the genetic factor most likely to bring about the individual difference in endurance capacity or its trainability. Platelets contain mtDNA but no nuclear DNA, whereas rho(0)-HeLa cells have nuclear DNA but no mtDNA. The oxidative capacity of mitochondria in the cultured cells, which were fused rho(0)-HeLa cell with platelets obtained from individual subjects (the so-called "cybrids"), reflects the individual mtDNA polymorphism in the gene-coding region. The purpose of this study was to investigate the relationship between the oxidative capacity of cybrids and the individual difference in endurance capacity, or its trainability. Forty-one sedentary young males took part in an 8-week endurance training program. They were determined by using their VO(2 max) as an index of endurance capacity on an ergocycle before and after the endurance training program. The relations between VO(2 max) before endurance training or the change of it by endurance training and the oxidative capacity of cybrids were investigated. There was no relation between them, and two groups were drawn from all subjects, based on one standard division of their initial VO(2 max): the higher pre-VO(2 max) group (n = 6) and the lower pre-VO(2 max) group (n = 5) (51.8 +/- 3.5 ml/min/kg vs. 33.3 +/- 3.8 ml/min/kg, p < 0.01). No significant difference was found between the O(2) consumption of the cybrids in the higher initial VO(2 max) group and that in the lower initial VO(2 max) group (16.3 +/- 4.9 vs. 15.9 +/- 2.0 nmol O(2)/min/10(7) cells, NS). Furthermore, neither the cytochrome c oxidase (COX) activity nor the complex I + III activity of cybrids showed a significant difference between the two groups. The oxidative capacity of cybrids between the high trainability group (n = 6) (Delta VO(2 max) 12.1 +/- 1.6 ml/min/kg) and the low trainability group (n = 9) (Delta VO(2 max) 2.3 +/- 0.5 ml/min/kg) was also similar. Thus the mtDNA polymorphism is very unlikely to relate to the individual difference in endurance capacity or its trainability in young sedentary healthy subjects.