Two cD-NAs coding for proximal tubular Na-Pi cotransport (NaPi-2) and Na-SO4 cotransport (NaSi-1) have been transfected by the use of a dexamethasone-inducible vector (pLK-neo) into MDCK and LLC-PK1 cells. By reverse transcription-polymerase chain reaction, expression of corresponding mRNAs was observed after stimulation with dexamethasone only. Similarly, expression of the NaPi-2 protein was detected only after induction with dexamethasone. In transfected Madin-Darby canine kidney (MDCK) cells, dexamethasone induced a large increase of Na-Pi or Na-SO4 cotransport, whereas, in transfected LLC-PK1, cell transport was only minimally expressed. In MDCK cells grown on filter supports, transfected Na-Pi-cotransport activity was equally expressed at both cell surfaces; dual location of expressed NaPi-2 protein was also observed by immunohistochemistry. In contrast, transfected Na-SO4 cotransport activity was predominantly expressed at the apical cell surface of MDCK cells. The results demonstrate that 1), in MDCK cells, the sorting behavior of two proximal tubular cotransport systems seems to be different: apical for Na-SO4 cotransport (NaSi-1) and dual location for Na-Pi cotransport (NaPi-2); and 2) LLC-PK1 cells seem not to be a suitable system to functionally express sodium-dependent cotransport systems for phosphate and sulfate.