Blood lactate concentration ([La⁻]_b) is one of the most often measured parameters during clinical exercise testing as well as during performance testing of athletes. While an elevated [Lai may be indicative of ischemia or hypoxemia, it may also be a “normal” physiological response to exertion. In response to “all-out” maximal exertion lasting 30–120 seconds, peak [La⁻]_b values of =15–25 mM may be observed 3–8 minutes postexercise. In response to progressive, incremental exercise, [La⁻]_b increases gradually at first and then more rapidly as the exercise becomes more intense. The work rate beyond which [La⁻]_b increases exponentially [the lactate threshold (LT)] is a better predictor of performance than VO_2max and is a better indicator of exercise intensity than heart rate; thus LT (and other valid methods of describing this curvilinear [La⁻]_b response with a single point) is useful in prescribing exercise intensities for most diseased and nondiseased patients alike. H+-monocarboxylate cotransporters provide the primary of three routes by which La⁻ transport proceeds across the sarcolemma and red blood cell membrane. At rest and during most exercise conditions, whole blood [La⁻] values are on average 70% of the corresponding plasma [La⁻] values; thus when analyzing [La⁻]_b, care should be taken to both (1) validate the [LaT-measuring instrument with the criterion/reference enzymatic method and (2) interpret the results correctly based on what is being measured (plasma or whole blood). Overall, it is advantageous for clinicians to have a thorough understanding of [La⁻] responses, blood La⁻ transport and distribution, and [La⁻] analysis.