Large losses of amino acids by diffusion were previously observed in Madin-Darby canine kidney (MDCK) cells during volume regulation. Also, an outward rectifying anion channel was activated. Because this channel was not selective among anions, it was suggested that it could be permeable to amino acids. Its permeability to aspartate, glutamate, and taurine was studied using the patch-clamp technique in the inside-out configuration. Solutions containing 500 mM aspartate or glutamate were used on the cytoplasmic side of excised patches to detect single-channel currents carried by these anions. Permeability ratios were estimated in two different ways: 1) from the shift in reversal potential of current-voltage curves after anion replacement in the bath solution and 2) from comparisons of amplitudes of single-channel currents carried by tested anions and chloride, respectively. The values of aspartate-to-chloride and glutamate-to-chloride permeability ratios obtained with both methods were quite consistent and were of the order of 0.2 for both amino acids. Taurine in solutions at physiological pH 7.3 is a zwitterionic molecule and bears no net charge. To detect single-channel currents carried by taurine, solutions containing 500 mM taurine at pH 8.2 were used in inside-out experiments. Under these conditions 120 mM of negatively charged taurine was present in the solutions bathing the cytoplasmic side of excised patches. The permeability ratio estimated from the shift in reversal potential was 0.75. These results showed that some of the organic compounds released by cells during regulatory volume decrease could diffuse through this outwardly rectifying anionic channel.