This study examined the influence of maturation on the oxygen uptake efficiency slope (OUES) in healthy male subjects. Seventy-six healthy male subjects (8-27 yr) were divided into groups based on maturation status: prepubertal (PP), midpubertal (MP), late-pubertal (LP), and young-adult (YA) males. Puberty status was determined by physical examination. Subjects performed a graded exercise test on a cycle ergometer to determine OUES. Group differences were assessed using a one-way ANOVA. OUES values (VO(2)L·min(-1)/log(10)V(E)L·min(-1)) were lower in PP and MP compared with LP and YA (p < .05). When OUES was expressed relative to body mass (VO(2)mL·kg-1·min(-1)/log(10)V(E)mL·kg(-1)·min(-1)) differences between groups reversed whereby PP and MP had higher mass relative OUES values compared with LP and YA (p < .05). Adjusting OUES by measures of body mass failed to eliminate differences across maturational groups. This suggests that qualitative factors, perhaps related to oxidative metabolism, account for the responses observed in this study.