TWIK1 is a member of the 2-P-domain potassium channel family and is strongly expressed in renal proximal tubules. In S2-3 segments, it is either localized in the subapical compartment or in the brushborder membrane. In immunofluorescence experiments we observed a striking gender difference for TWIK1: In proximal tubules (S2-3) of male mice, TWIK1 was mainly localized in the subapical vesicles which were distinct from endocytotic vesicles as shown by FITC-dextran endocytosis. In females, TWIK1 fluorescence is 2 to 3-fold higher and localized mainly to the brushborder membrane of S2-3 segments. In addition to the gender difference in protein abundance, TWIK1 mRNA was increased in females as measured by realtime PCR. Treatment of female mice with testosterone for seven days led to a male-like distribution of TWIK1 in the kidney. Prolactin, progesterone, and estradiol did not change localization and abundance of TWIK1 in male mice. Interestingly, TWIK1 largely co-localized with the glucose transporter SGLT1 in S2-3 proximal tubules. This is in agreement with a recent study showing similar gender differences for SGLT1 in rats. These data suggest a functional coupling of SGLT1 and TWIK1: Presumably TWIK1 counteracts SGLT1-induced depolarization thereby stabilizing the driving force for ongoing electrogenic transport.