To determine whether plasma lactate can be a significant fuel for human brain energy metabolism, infusions of [3-¹³C]lactate and ¹H-¹³C polarization transfer spectroscopy were used to detect the entry and utilization of lactate. During the 2 h infusion study, ¹³C incorporation in the amino acid pools of glutamate and glutamine were measured with a 5 min time resolution. With a plasma concentration ([Lac]_P) being in the 0.8–2.8 mmol/L range, the tissue lactate concentration ([Lac]_B) was assessed as well as the fractional contribution of lactate to brain energy metabolism (CMRlac). From the measured relationship between unidirectional lactate influx (V_in) and plasma and brain lactate concentrations, lactate transport constants were calculated using a reversible Michaelis–Menten model. The results show that (1) in the physiological range, plasma lactate unidirectional transport (V_in) and concentration in tissue increase close to linearly with the lactate concentration in plasma; (2) the maximum potential contribution of plasma lactate to brain metabolism is 10% under basal plasma lactate conditions of ∼1.0 mmol/L and as much as 60% at supraphysiological plasma lactate concentrations when the transporters are saturated; (3) the half-saturation constant K_T is 5.1 ± 2.7 mmol/L and V_MAX is 0.40 ± 0.13 μmol · g⁻¹ · min⁻¹ (68% confidence interval); and (4) the majority of plasma lactate is metabolized in neurons similar to glucose.