As a wide range of values have been suggested for the relative energetics of 400-m and 800-m track running events, this study aimed to quantify the respective aerobic and anaerobic energy contribution to these events during actual track running. Sixteen trained 400-m (11 male, 5 female) and 11 trained 800-m (9 male and 2 female) athletes acted as subjects for this study. Subjects performed (on separate days) a laboratory graded exercsie test and multiple race time trials. The relative energy system contribution was calculated by multiple methods based upon measures of race O2, Accumulated Oxygen Deficit (AOD), blood lactate and estimated phosphocreatine degradation (La/PCr). Aerobic ' anaerobic energy system contribution (AOD method) to the 400-m event was calculated as 41% - 59% (male) and 45% - 55% (female). For the 800-m event, an increased aerobic involvement was noted with a 60% - 40% (male) and 70% - 30% (female) respective contribution. Significant (p<0.05) negative correlations were noted between race performance and anaerobic energy system involvement (La/PCr) for male 800-m and female 400-m events (r = -0.77 and -0.87 respectively). This data collected during actual track running compares well with previous estimates of the relative energy system contributions to 400-m and 800-m events. Additionally, the relative importance and speed of interaction of the respective metabolic pathways has implications to training for these events.