The mechanism of the mouse (m)B⁰AT1 (slc6a19) transporter was studied in detail using two electrode voltage-clamp techniques and tracer studies in the Xenopus oocyte expression system. All neutral amino acids induced inward currents at physiological potentials, but large neutral non-aromatic amino acids were the preferred substrates of mB⁰AT1. Substrates were transported with K_0.5 values ranging from approx. 1 mM to approx. 10 mM. The transporter mediates Na⁺–amino acid co-transport with a stoichiometry of 1:1. No other ions were involved in the transport mechanism. An increase in the extracellular Na⁺ concentration reduced the K_0.5 for leucine, and vice versa. Moreover, the K_0.5 values and V_max values of both substrates varied with the membrane potential. As a result, K_0.5 and V_max values are a complex function of the concentration of substrate and co-substrate and the membrane potential. A model is presented assuming random binding order and a positive charge associated with the ternary [Na⁺–substrate–transporter] complex, which is consistent with the experimental data.