Carbamazepine (CBZ) and oxcarbazepine are frequently used antiepileptic drugs. A side effect of both drugs in therapy is hyponatremia with an incidence of up to 48%. Most of the cases remain asymptomatic. The mechanism of drug action on epithelial salt and water transport is unclear. In vivo studies in rat indicated an effect of CBZ on epithelial ion transport. We characterized the effect of CBZ in rat distal colon. Na⁺ transport via the epithelial Na⁺ channel ENaC remained unaffected. Clsecretion in distal colon depends on luminal CFTR activation and on the presence of at least one basolateral K⁺ conductance, either cAMP dependent or Ca²⁺ activated. Basolateral application of CBZ reduced cAMP-stimulated Cl^-secretion (84% at 300mM) in a concentration dependent manner (IC₅₀: 26mM). In the continuous presence of cAMP stimulation CBZ (100mM) also inhibited the effect of carbachol (100mM) which acts via intracellular Ca²⁺ activated K⁺ channels. These findings suggest most likely an effect on luminal Cl^-conductance or on its activation via cAMP. In addition, under cAMP stimulated conditions initial addition of CBZ led to a short and transient increase in transepithelial equivalent short-circuit current. This could be explained by a transient increase in intracellular Ca²⁺ and a slower onset of Cl^-channel inhibition by targeting intracellular cAMP signalling. In conclusion our data show a new yet unknown aspect of CBZ action on epithelial function.