The purpose of this study was to establish the reliability of lactate threshold (LT) measurement across days. Twelve endurance-trained males aged 21-44 years participated in the study. Blood lactate concentration in capillary whole blood was measured at rest and at completion of 5 minute stages on a treadmill at 60%, 65%, 71%, 77%, 83% and 89% of VO2max during 3 separate days of testing. Velocity at LT (LT(VEL)), oxygen consumption at LT (LT(VO2)), and heart rate at LT (LT(HR)) are determined using each of the following criteria: fixed lactate concentrations of 2.0 mmol, 2.5 mmol, and 4.0 mmol, the breakpoint of the lactate-intensity curve, 1 mmol above rest, and 1 mmol above exercise baseline. A one factor repeated measures ANOVA model was used to calculate intraclass reliability coefficients across the 3 testing sessions. Confidence intervals (95%) were calculated around the point estimates of reliability for each LT criterion. Intraclass correlation coefficients ranged from 0.98 to 0.99 LT(VEL) (SEM 0.06 to 0.14 km x hr(-1)), from 0.91 to 0.96 for LT(VO2) (SEM 0.34 to 0.72 ml x kg(-1) x min(-1)), and from 0.75 to 0.96 for LTHR (SEM 0.7 to 1.9 beats x min(-1)). These results support the reliability of LT(VEL), LT(VO2), and LT(HR) measurement.