Transport of taurine has been demonstrated in capillary preparations from adult rat brains using [3H]taurine. Taurine transport is mediated by a saturable high-affinity system which is entirely dependent on sodium ions. The apparent maximal influx (Vmax) and half-saturation concentration (Km) corresponded to 1.06.10(-4) mumol/min per mg protein and 27.5 microM, respectively. Competition experiments in the presence of sodium ion showed that [3H]taurine uptake was strongly inhibited by 0.1 mM unlabeled structural analogues of taurine such as beta-alanine and hypotaurine as well as unlabeled taurine. gamma-Aminobutyric acid (GABA) (0.1 mM) inhibited the uptake of labeled taurine by 30%, whereas isethionic acid, L-methionine, L-2,4-diaminobutyric acid, glycine, L-cysteinesulfonic acid and cystamine did not exhibit any inhibitory effect. The results suggest that the Na+ gradient is the principal source of energy for taurine transport into isolated brain capillaries. This transport system may play an active role in the regulation of taurine concentration in the brain extracellular space.