At any given running speed, weight support with a lower body positive pressure (LBPP) device (i.e. Alter-G treadmill) reduces V̇O2. However, it is unknown how application of LBPP during running impacts lactate metabolism, specifically lactate threshold. Purpose: To determine if body weight unloading with the Alter-G treadmill alters lactate threshold. Methods. Maximal aerobic capacity (V̇O2max) and lactate threshold (LT) was determined in 8 male subjects on an Alter-G treadmill at 100% and 80% body weight loading at 0% grade in a randomized crossover design. V̇O2max tests started at 7 mile h-1 and increase speed by 1 mile h-1 every 2 min till voluntary exhaustion and were separated by a minimum of 7 days. LT tests started at 5 mile h-1 and increased speed to 6, 7, 7.5, 8.0, 8.5, 9.0 (additional stages increase speed by 0.5 mile h-1) every 3 min until the subject reached ¡Ö85% of V̇O2max. LT tests were separated by a minimum 3 days. V̇O2, heart rate (HR), mean arterial blood pressure (MAP) and changes in Hct, [Hb], and total protein ([TP]) were determined on separate days in a randomized crossover design. Plasma lactate concentrations were determined from venous blood samples (4 ml) obtained at rest and during the last minute of each exercise stage. Lactate threshold was determined from a log-log plot of lactate concentration (mM) and relative V̇O2 (ml O2 min-1 kg-1 BM). Results. V̇O2max determined during running at 100% and 80% loading were similar (52.3 ± 0.9 and 52.7 ± 0.7 ml O2 min-1 kg-1 BM, respectively). The energy cost of running at 9 mile h-1 (all subjects completed stages between 5 and 9 mile h-1) was reduced by 12% at 80% body weight (37.2 ± 2.9 ml O2 min-1 kg-1 BM) compared to running at 100% body weight (42.3 ± 1.7 ml O2 min-1 kg-1 BM, <0.05). However, plasma lactate at 9 mile h-1 was similar during 80% and 100% body weight running (3.4 ± 0.4 and 3.1 ± 0.7 mM, respectively). Plasma lactate at a given V̇O2 was higher (p < 0.05) while running at 80% body weight compared to 100% body weight running. Calculated LT at 100% BW loading (36.3 ± 1.3 ml O2 min-1 kg-1 BM) was higher than 80% BW loading (32.2 ± 1.8 ml O2 min-1 kg-1 BM, p<0.05). During running at 80% BW HR was reduced compared to 100% BW running (p<0.05) however the MAP response was similar. During exercise the reduction in PV, at any given V̇O2 was larger at 80% BW compared to 100% BW running (p<0.001). Conclusion. During running, BW unloading with LBPP decreased the energy cost of exercise but not lactate levels. Body weight unloading caused a lowering of the LT. The reduction in whole body energy cost was not associated with a reduction in the lactate production since plasma lactate accumulation at a given speed was similar with and without LBPP.