Evidence shows that lactate threshold (LT) is a valid tool to evaluate endurance capacity and is used to prescribe training intensities. However, there are discrepancies between LT test methodologies and the way to use LT to prescribe training velocities, maintaining metabolic stress in steady-state intensity bouts. Few studies have investigated the relationship of LT Velocity at 4 mMol/l (V4) with intensity prescription on Interval Training (IT) workouts. PURPOSE: To determine if V4, obtained via an incremental test, can maintain Blood Lactate (BL) predicted, during a steady-state IT workout in swimmers. METHODS: Ten well trained swimmers (19.42±6.77 yrs) performed two freestyle tests: an incremental 6x200-m test with 1-min passive rest, measuring Heart Rate (HR) and BL after each repetition to determine V4; and, 4 days after, a steady-state IT 10x200-m test with 1-min passive rest at V4, measuring BL and HR after reps. 2-4-6-8-10. Paired t-tests were used to compare V4 vs. IT speeds and times and [email protected] vs. BL after reps. 2-4-6-8-10. In addition, repeated measures ANOVA was used to compare BL after reps. 2-4-6-8-10. Finally, Pearson's correlations (r) were obtained between BL vs HR in both incremental and steady state tests. RESULTS: Same speeds and times for V4 and IT were observed (1.38±0.07 m/s and 145.4±7.6 s vs 1.38±0.07 m/s and 145.5±7.5 s, respectively, p>0.05). BL levels were maintained at [email protected] levels only during reps 2 and 4 (4.10±0.52; 3.72±0.63 mMol/l, respectively, p>0.05 vs. [email protected]); however, BL levels decreased over time during reps 6, 8, and 10 (3.59±0.29; 3.40±0.33; 3.13±0.30 mMol/l, respectively, p<0.01 vs. [email protected] and vs. reps 2 and 4). Additionally, there was a moderate correlation (r = 0.69) between BL and HR during the incremental test. However, a low correlation (r = 0.26) between BL and HR during the steady state IT test was observed. CONCLUSION: V4 from an incremental test underestimates BL level showing, a progressive decrease during the steady-state IT test. These results suggest that IT at V4 might not be enough to maintain metabolic stress during an IT bout, especially during the second half of it. Additionally, the moderate and low correlations observed between BL vs. HR suggest that HR might not be a good marker of exercise intensity in swimmers.