We report the measurement of d-β-hydroxybutyrate (BHB) in the brains of six normal adult subjects during acute infusions of BHB. We used high field in vivo1H magnetic resonance (MR) spectroscopy in the occipital lobe in conjunction with an acute infusion protocol to elevate plasma BHB levels from overnight fasted levels (0.20 ± 0.10 mm) to a steady state value of 2.12 ± 0.30 mm. At this level of hyperketonemia, we determined a tissue BHB level of 0.24 ± 0.04 mm. No increases in brain lactate levels were seen in these data. The concentrations of BHB and lactate were both considerably lower in comparison with previous data acquired in fasted adult subjects. This suggests that up-regulation of the monocarboxylic acid transporter occurs with fasting.