Interlobular ducts were isolated from the pancrease of copper-deficient rats and maintained in culture on polycarbonate filter rafts. Within 8 h the ends of the ducts had sealed. This was followed by a marked dilatation of the lumen, a flattening of the epithelium against the surrounding connective tissue layer and an over-all swelling of the duct. Apart from a reduction in their height, a fall in the number of intracellular fat droplets and a widening of intercellular spaces, epithelial cells within the cultured ducts retained all the ultrastructural characteristics of those in freshly isolated preparations. The basal concentration of adenosine 3',5'-phosphate (cyclic AMP) in the cultured ducts was 43.3 +/- 6.8 mumol.1-1 duct epithelium (n = 5) and was increased to 188.9 +/- 55.2 mumol.l-1 duct epithelium (n = 5) in the presence of 10(-8) M secretin. The basal rate of fluid secretion, measured using micropuncture techniques, was 0.16 +/- 0.03 nl.h-1.nl-1 duct epithelium (n = 12). This was increased 14-fold by 10(-8) M secretin while the dose of the hormone required for half-maximal secretion was about 2 X 10(-11) mol.l-1. The concentration of chloride ions in secreted fluid and perifusion buffer were similar. Variation in culture time up to 52 h had no effect on fluid secretion, and the response to secretin was dependent on the presence of bicarbonate ions in the perifusion fluid. N6,O2-dibutyryl adenosine 3',5'-phosphate (dibutyryl cyclic AMP) also increased fluid secretion but caerulein had no effect. We suggest that isolated ducts secrete fluid at comparable rates to ducts in situ within the pancrease of copper-replete rats.