The aim of this study was to examine the criterion validity of 2 lactate thresholds (LTs, intensity corresponding to 1 mmol·L(-1) above baseline; onset of blood lactate accumulation, intensity at 4 mmol·L(-1)) determined with a fixed-distance incremental field test by assessing their correlation with those obtained using a traditional fixed-time laboratory protocol. A second aim was to verify the longitudinal validity by examining the relationships between the changes in LTs obtained with the 2 protocols. To determine the LTs, 12 well-trained male middle and long distance amateur and competitive runners training from 4 to 7 d·wk(-1) (age 25 [5] years, body mass 66 [5] kg, estimated VO(2)max 58.6 [4.9] ml·min(-1)·kg(-1), SD in parentheses) performed in 2 separate sessions an incremental running test on the field starting at 12 km·h(-1) and increasing the speed by 1 km·h(-1) every 1,200 m (FixD test) and an incremental treadmill test in the laboratory starting at 12 km·h(-1) and increasing the speed by 1 km·h(-1) every 6 minutes. The 2 tests were repeated after 6-12 weeks. A nearly perfect relationship was found between the running speeds at LTs determined with the 2 protocols (r = 0.95 [CI95% 0.83-0.99]; p < 0.001). The correlations between longitudinal changes in LTs were very large (0.78 [0.32-0.95; p = 0.006]). The heart rate corresponding to the LTs were not significantly different. This study showed the criterion and longitudinal validity of LTs determined with a protocol consisting of fixed-distance intervals performed in field setting.