The major K influx pathways and their response to thiol modification by N-ethylmaleimide (NEM) and protein kinase and phosphatase inhibitors were characterized in human lens epithelial B3 (HLE-B3) cells with Rb as K congener. Ouabain (0.1mM) and bumetanide (5μM) discriminated between the Na/K pump (∼35% of total Rb influx) and Na–K–2Cl cotransport (NKCC) (∼50%). Cl-replacement with nitrate or sulfamate revealed 100μM, activated the Na/K pump and abolished NKCC but did not affect KCC. The data suggest at least partial inverse regulation of KCC and NKCC in HLE-B3 cells by signaling cascades involving serine, threonine and tyrosine phosphorylation/dephosphorylation equilibria.