Cell-attached recordings revealed Cl− channel activity in basolateral membrane of guinea pig distal colonic crypts isolated from basement membrane. Outwardly rectified currents (gpClor) were apparent with a single-channel conductance (γ) of 29 pS at resting membrane electrical potential; another outward rectifier with γ of 24 pS was also observed (∼25% of gpClor). At a holding potential of −80 mV γ was 18 pS for bothgpClor currents, and at +80 mV γ was 67 and 40 pS, respectively. Identity as Cl− channels was confirmed in excised patches by changing bath ion composition. From reversal potentials, relative permeability of K+ over Cl− (P K/PCl) was 0.07 ± 0.03, with relative permeability of Na+over Cl−(P Na/P Cl) = 0.08 ± 0.04. A second type of Cl− channel was seen with linear current-voltage (I-V) relations (gpClL), having subtypes with γ of 21, 13, and 8 pS. Epinephrine or forskolin increased the number of opengpClor and gpClL. Open probabilities (P o) ofgpClor, gpClL21, andgpClL13 were voltage dependent in cell-attached patches, higher at more positive potentials. Kinetics ofgpClor were more rapid with epinephrine activation than with forskolin activation. Epinephrine increasedP o at the resting membrane potential forgpClL13. Secretagogue activation of these Cl−channels may contribute to stimulation of electrogenic K+ secretion across colonic epithelium by increasing basolateral membrane Cl− conductance that permits Cl− exit after uptake via Na+-K+-2Cl− cotransport.