Data are presented in support of the transport of (-)-D-3-hydroxybutyrate across the blood-brain barrier (BBB) being a carrier-mediated process. The kinetic parameters in 21-day-old pentobarbital-anaesthetized rats were Vmax 2.0 mumol.g-1.min-1, Km 29 mM, and KD 0.024 ml.g-1.min-1. The value for Vmax was the same as that for L-lactate and pyruvate transport in animals of the same age. The transport of all three substrates was sensitive to inhibition by low concentrations of either 2-oxo-3-methylbutanoate or 2-oxo-4-methylpentanoate, the 2-oxo acids that can accumulate in patients with maple-syrup-urine disease. The Ki values for the 2-oxo acids were severalfold lower than the respective Km values. 2-Oxo-3-phenylpropionate was a poor inhibitor. The relative affinities of the various monocarboxylic acids for the transport system of the BBB distinguished it from similar systems described in brain, heart, and liver mitochondria; human erythrocytes; and Ehrlich ascites-tumour cells.