The relationship between relative aerobic power (rel.VO(2)max) as a generally accepted indicator of endurance capacity and certain characteristics of the athlete's heart, such as body-size related (relative) left ventricular (LV) diastolic wall thickness (WTd), internal diameter (LVIDd), muscle mass (MM), WTd/IDd, heart rate (HR), fractional shortening (FS) and E/A ratio, were investigated in 346 young males (18-35 years, 291 athletes of various events and 55 nonathletic control subjects). Rel.VO(2)max was measured by spiroergometry; cardiac characteristics were determined by two-dimensionally guided M-mode and Doppler-echocardiography. When the groups were pooled, correlation of rel.VO(2)max with the cardiac parameters was significant: LVMM.BSA(-1.5)= 0.413, LVWTd.BSA(-0.5)= 0.327, LVIDd.BSA(-0.5)= 0.292, HR =-0.434, E/A = 0.272 (P < 0.001), but no significant relationship was seen with FS and WTd/IDd. In the endurance trained group, rel. VO(2)max correlated significantly with LVMM.BSA(-1.5), LVWT.BSA(-0.5), HR, and E/A, in the ballgame players with LVMM.BSA(-1.5), LVWT.BSA(-0.5), and E/A, in the power-and-sprint event athletes with HR and E/A. In the control group, no significant relationship was observed. Results indicate that in athletes having higher endurance capacity maximal oxygen consumption depends largely on cardiac condition, while in athletes with a lower endurance capacity it can be limited by peripheral conditions.