In neuroblastoma x glioma hybrid cells, a cell line of neuronal character, a saturable uptake system for taurine is found which displays high affinity and high capacity (km = 38 micro M, V = 1.25 nmol . mg-1 . min-1). Only the closely related structural analogues hypotaurine and beta-alanine are able to inhibit the transport of radioactively labeled taurine. Imipramine or haloperidol at 100 micro M effectively blocks taurine uptake. High-affinity taurine uptake shows an absolute and highly specific requirement for Na+. The hybrid cells internalize taurine very slowly and, with 1 mM extracellular taurine, attain a plateau only after more than 20 h, at which time approximately 34 mM labeled taurine has accumulated in the cytosol. Generally there is hardly any spontaneous release of accumulated taurine. Efflux can, however, be induced by increasing the intracellular Na+ content and is then accelerated by lowering the extracellular Na+ concentration. The hypothesis is forwarded that taurine may exert its function by driving the extrusion of Na+ in emergency situations.