The capacity of papillary cells to adapt to elevated osmotic concentrations is unusual among mammalian cells. This capacity was evaluated by using primary tissue culture. Viability and growth of cells in rat renal papillary tissue explants were assessed after culture in media adjusted with urea and sodium chloride to various osmotic concentrations between 300 and 1,500 mOsm/kg water. The survival of cells, including cells resembling those of the collecting ducts and the loop of Henle, was greatest in medium adjusted to 1,000 mOsm with equiosmolar amounts of the two solutes. At 1,500 mOsm only cuboidal tubular epithelium resembling collecting duct epithelial cells survived. In contrast, cells of cortical tissue survived and grew at 300 and 640 mOsm, but not at 1,000 mOsm or above. Epithelial monolayers appeared to proliferate from collecting ducts and spread over the surface of the explants as well as onto the glass surface in the culture dish. Epithelial growth of medullary tissue was most rapid at 300 mOsm and was slower at 700 and 1,000 mOsm. Monolayers did not form at 1,500 mOsm; however, epithelial overgrowth of explants did occur. Hydropenia in the donor animal did not significantly affect the viability or growth of cultured papillary tissue. Explants cultured for 5 days at 300 mOsm followed by a stepwise increase in medium osmolality to 1,100 or 1,500 mOsm and cultured for 3 more days showed low or no survival whereas explants cultured at 700 mOsm survived such increases. Explants cultured for 5 days at 1,500 mOsm survived and grew monolayers when lowered to 300 mOsm. Poor viability and no epithelial proliferation were observed in explants cultured in medium adjusted to 900 mOsm with either urea or sodium chloride alone, suggesting that a mixture of the two solutes in the extracellular space, as found in vivo, may be essential in achieving elevated osmolalities.