The effects of secretin, the physiological secretagogue for pancreatic ducts, were studied in CAPAN-1 pancreatic duct carcinoma cells. When grown to confluence on plastic dishes, CAPAN-1 cells form domes and exhibit marked increases in culture content of Na+ and urea distribution space (UDS). This parameter is measured as an index of both intracellular and dome compartments under the conditions adopted. Both Na increase and dome formation are inhibited by long term incubation with phorbols, DIDS, DPC, EIPA, H2DIDS, and brefeldin. Short term treatment with secretin or 8-Br-cAMP/teophylline causes dome collapse and a marked decrease in UDS and culture content of Na. Secretin-induced sodium decrease is not abolished by ion channel inhibitors, suggesting that diffusion routes other than ion channels are involved in hormone effects. This hypothesis is also in agreement with data obtained on CAPAN-1 cells cultured on permeable inserts, where no change in Na content or UDS is detected upon secretin treatment. Confluent monolayers exhibit a high transepithelial resistance (Rms) which is markedly and reversibly decreased by secretin. The hormone also decreases the transepithelial voltage (Vms) and raises the monolayer permeability to mannitol. It is concluded that secretin enhances the paracellular permeability of pancreatic duct cells. This effect of secretin, unknown thus far, may be involved in the mechanism of pancreatic secretion in vivo.