We have studied the modulation and pharmacological properties of two anion channels in T, ceils by recording single channel and transepithelial currents. One channel had an outwardly rectifying current-voltage Z/V curve, was rarely active in cell-attached patches, and was unaffected by CAMP. The other channel had lower conductance (8.7 pS at 37°C) and a more ohmic Z/V relationship. Exposure to CAMP increased the probability of observing low-conductance channel activity in cell-attached patches > 6-fold. Extracellular DIDS (4,4'-diisothiocyanatostilbene-2,2'disulfonic acid) or IAA-94 (an indanyloxyacetic acid) inhibited the outward rectifier but did not a&t the low-conductance channel or CAMP-stimulated transepithelial current. These results suggest the low-conductance Cl channel may contribute to apical membrane conductance during CAMP-stimulated secretion.