BACKGROUND: Important isotope studies related to lactate metabolism have pointed out that blood lactate level (BLa) is an expression of the balance between Production (RProd), Removal (Rt) and Oxidation (ROx) rates. Also, evidence shows that the relationship between BLa with Rt and ROx rates, is concentration-dependent, close 4 to 7 mmol/l. PURPOSE: Analyze BLa in the first sec. (BLa-10s) and last sec. (BLa-50s) of 1-min passive rest, during steady-state moderate intensity interval training (IT), considering that BLa is a balance of RProd with Rt and ROx intracellular process. We want to demonstrate that (BLa-10s) and (BLa-50s) levels are similar, so Rt and ROx are maintained, in steady-state conditions, during IT workouts. METHODS: Ten trained swimmers and eight trained track athletes (19.1±2.6 yr) performed a moderate intensity IT session (BLa: 4 to 7 mmol/l). Swimmer´s protocol was: 10 x 100m, with 60s rest; and Athlete’s protocol was: 10 x 400m, with 60s rest. BLa during BLa-10s and BLa-50s were measured with Lactate Plus meter and heart rate (HR) was monitored. Measurements were collected every 2 reps. We compared BLa-10s vs. BLa-50s applying paired t-test with a p<0.05 level, and r between BLa-10s vs. BLa-50s, and BLa-10s vs. HR, in reps. 2-4-6-8 and 10. RESULTS: We found no significant differences between BLa-10s and BLa-50s. BLa-10s ranged 5.05±1.51 - 6.61±1.10 mmol/l and BLa-50s ranged 5.03±1.80 - 6.71±1.07 mmol/l. Additionally, we found a low r between BLa-10s and HR values (r = -0.06). However, a high r between BLa-10s and BLa50-s values (r = 0.94) was observed. CONCLUSION: BLa-10s and BLa-50s showed same metabolic steady-state conditions. These results support the evidence that moderate intensity IT produces physiological stress during the whole workout duration (exercise / rest). Also, we determine that HR is not a valid variable to control for physiological demands during IT. Keyword: Blood Lactate; Endurance Training; Metabolic Stress