The expression of the Na+-K+-2Cl- cotransporter (NKCC1) in alpha cells and beta cells from the rat pancreas was examined. Isolated alpha cells and beta cells in a mixed islet cell preparation were identified by volume using video-imaging methods, and by the expression of glucagon or insulin. The expression of mRNA for NKCC1 in pancreatic islets was demonstrated by RT-PCR. Immunocytochemical studies showed that the NKCCI protein was expressed in rat beta cells, but not alpha cells. The activity of Na+-K+-2Cl- cotransporter was also examined, by studying cell volume regulation in response to HEPES-buffered, hypertonic solutions. A regulatory volume increase was observed in the beta cells but not the alpha cells. It is concluded that the NKCC1 is expressed in rat pancreatic beta cells but not alpha cells. This is consistent with the hypothesis that Cl- is accumulated above the expected equilibrium distribution in beta cells, but is below equilibrium in alpha cells.