The electroneutral potassium chloride co-transporter KCC2 is the major chloride extruder in mature neurons. During development upregulation of KCC2 establishes hyperpolarizing GABAergic synaptic inhibition. It has been shown previously that the protein contains several putative phosphorylation sites which regulate different aspects of KCC2 function. Despite the knowledge of these sites of regulation a comparison of the efficacy of these regulatory sites on transport activity in neurons and how this is transduced into strength of GABAergic inhibition is missing. Therefore we used NH₄⁺ induced pH_i shifts to quantify KCC2 transport activity and gramicidine perforated patch recordings to determine E_GABA in cultured hippocampal neurons transfected with KCC2 mutants in which threonines have been replaced by alanine. Our data show that KCC2 can reach different levels of transport activity depending on the site of mutation. Simultaneous mutation of multiple threonines to alanine resulted in a further increase of transport activity compared to single mutations. E_GABA depended linearly on the V_max of KCC2 transport activity. Hill Plots derived from the concentration dependence of NH₄ induced pH_i shifts for different mutants suggest cooperativity for KCC2 mediated NH₄ uptake. In summary a shift in the membrane potential to more negative values demands an increase in KCC2 transport activity to maintain hyperpolarizing inhibition. The KCC2 protein contains multiple sites through which its transport activity could be adapted.