In the adult brain, chloride (Cl − ) influx through GABA A receptors is an important mechanism of synaptic inhibition. However, under a variety of circumstances, including acquired epilepsy, neuropathic pain, after trains of action potentials or trauma, and during normal early brain development, GABA A receptor activation excites neurons by gating Cl − efflux because the intracellular Cl − concentration (Cl i ) is elevated. These findings require an inducible, active mechanism of chloride accumulation. We used gramicidin-perforated patch recordings to characterize Cl − transport via NKCC1, the principal neuronal Cl − accumulator, in neonatal CA1 pyramidal neurons. NKCC1 activity was required to maintain elevated Cl i such that GABA A receptor activation was depolarizing. Kinetic analysis of NKCC1 revealed reversible transmembrane Cl − transport characterized by a large maximum velocity ( v max ) and high affinity ( K m ), so that NKCC1 transport was limited only by the net electrochemical driving force for Na + , K + , and Cl − . At the steady-state Cl i , NKCC1 was at thermodynamic equilibrium, and there was no evidence of net Cl − transport. Trains of action potentials that have been previously shown to induce persistent changes in neuronal E Cl (reversal potential for Cl − ) did not alter v max or K m of NKCC1. Rather, action potentials shifted the thermodynamic set point, the steady-state Cl i at which there was no net NKCC1-mediated Cl − transport. The persistent increase in Cl i required intact α2/α3 Na + -K + -ATPase activity, indicating that trains of action potentials reset the thermodynamic equilibrium for NKCC1 transport by lowering Na i . Activity-induced changes in Na + -K + -ATPase activity comprise a novel mechanism for persistent alterations in synaptic signaling mediated by GABA.