Potassium secretion along the renal collecting duct plays a key role in the body fluid potassium homeostasis. ROMK K+ channels (Kir1.1) cloned from rat kidney by Hebert’s group (1) are the major candidate for the route of this secretion. To date, several investigators have demonstrated the regulation of functional expression of transfected ROMK K+ channels in various cells (2, 3, 4). However, little is known about the affinity of the functional expression of ROMK K+ channels to the specific cell membrane. Recently, we have cloned ROMK1 (Kir1.1a) K+ channel and examined the exogenous expression of the cloned ROMK1 fused with EGFP in polarized and non-polarized membranes of cultured mouse collecting duct (M1) cells. We used two types of cellular conditions in culture. One was single M1 cells on conventional glass dishes, and the other was confluent M1 cells on membrane inserts to expose the polarized apical membrane of the cells. Visual expression of ROMK-EGFP in the cells was confirmed with a fluorescent microscope, and the functional expression was examined using the cell-attached and the whole-cell modes of the patch-clamp technique. Values are mean ± S.E.M., compared by ANOVA. Without the exogenous transfection, ROMK-like K+ channel has not been detected in both single and confluent cells. The lack of intrinsic ROMK in M1 cells was also confirmed by Western blot. The current identical to ROMK1 K+ channel was observed in the ROMK-EGFP-expressing M1 cells in both single cells on glass and confluent cells on membrane inserts in cell-attached patches. Although no appreciable difference in fluorescence intensity and the localization of ROMK-EGFP was observed between the two conditions, there was a significant difference of the frequency of the ROMK current acquisition. Namely, active ROMK1 K+ channels were detected in 67.9 % of the confluent cells and in 15.6 % of single cells. Numbers of active ROMK1 K+ channels in individual patch-membranes of the confluent cells and the single cells were 2.21±0.44 (n=28) and 0.40±0.17 (n=45), respectively, indicating that expression of ROMK1 K+ channel activity in the confluent cells is significantly higher than that in the single cells in cell-attached patches (p<0.01). Moreover, whole-cell current of the single cells with transfected ROMK-EGFP was not affected by K+ channel blocker, Ba2+ (2 mM), but that of the confluent cells with ROMK-EGFP was markedly attenuated by Ba2+. From the above data, it is strongly suggested that the exogenously transfected ROMK1 K+ channel has a high affinity to the polarized apical membrane to express its activity in cultured M1 cells. The factor for this affinity remains to be investigated.