Taurine secretion by the flounder kidney is dependent on external Cl− at the basal lateral membrane. Although taurine uptake by renal BBMV in mammalian species is Na+-dependent (Rozen et al., Biochem J 180:245, 1979), little is known about the role of Cl− on the accumulations of this β-amino acid. Of all anions tested; Cl, NO3, SO4, SCN, and tartarate, a typical overshoot was noted only in the presence of an NaCl gradient. Preincubation with choline Cl resulted in reduced uptake after imposition of an external NaCl (100mM) gradient. Since choline is poorly permeant, BBMV were preloaded with a series of Li+ salts prior to incubation in 100 mM NaCl. Licl preincubation reduced taurine uptake by 40-60%, p<.001; whereas preincubation in LiNO3 or LiSCN did not inhibit. With LiSO4 preincubation taurine uptake was actually stimulated at 15, 60 and 360 sec.; at equilibrium, (45 min) this Cl effect was not apparent. Nigericin in the presence of external KCl stimulated uptake by 30%, p<.001, but reduced uptake when KCl was inside, p<.01. Gramicidin, which makes membranes more permeant to Na, significantly reduced taurine uptake in the presence of NaCl, but had no effect in its absence. These findings are consistent with the hypothesis that internal Cl− inhibits and that external Cl− is required to fully demonstrate the Na+-dependent uptake of the amino acid taurine and indicates that the anionic species present may govern amino acid transport by mammalian kidney.