Madin-Darby canine kidney (MDCK) cells, when seeded into collagen gel, from polarized, spherical, epithelial cysts, which grow by a process involving fluid secretion and cell proliferation. These cysts are a useful model for understanding the dynamics of cyst enlargement in renal cystic disease. The hypothesis that MDCK cyst fluid secretion depends upon chloride secretion was tested, and a cell model for this process is presented here. Lumen and epithelial cell volumes were measured by video microscopy in acute experiments. Fluid absorption (-0.073 +/- 0.007 microliters.h-1.cm-2; N = 8) was observed when cysts were superfused with unsupplemented Dublecco's modified Eagle's medium at 36 to 37 degrees C. Fluid secretion (0.221 +/- 0.0016 microliters.h-1.cm-2; N = 25) was seen when 1 mM dibutyryl cAMP plus 0.1 mM 3-isobutyl-1-methylxanthine were added to the superfusate. cAMP-induced fluid secretion was significantly inhibited by basolateral 1 mM ouabain, 0.1 mM furosemide, or 1 mM amiloride. It was not significantly affected by 1 mM chlorothiazide, 0.01 mM bumetanide, or 0.1 mM acetazolamide in the presence of normal bicarbonate/CO2. In the nominal absence of bicarbonate/CO2 fluid secretion was 18% of control. Vasopressin-induced fluid secretion was significantly inhibited by pretreatment of cysts with 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Cyst cell shrinkage in isosmotic chloride-free Ringer's solution (chloride replaced by gluconate) was inhibited by 0.1 mM basolateral DIDS. The results suggest that chloride-bicarbonate exchange in the basolateral membrane of MDCK cyst epithelial cells plays a critical role in cyst fluid secretion.