Endothelial cells of brain microvessels constitute a tight barrier between the blood and the brain. They also secrete the interstitial fluid of the brain, Na⁺ transport being principally by the serial action of an NKCC contransporter on the luminal membrane (O’Donnell et al, 2004) and the Na-K-ATPase on the abluminal membrane (Betz, 1983). This model of secretion suggests that K⁺ is recycled on both membranes of the cell. Indeed, brain endothelial cells have a high permeability to radiolabelled K⁺ (Smith & Rapoport, 1986), although little is known about the mechanisms of K⁺ transport that the above model requires. Thus, we have investigated conductive K⁺ transport in brain endothelial cells. Endothelial cells from brain microvessels were isolated from rats, killed in accordance with UK legislation. Cells were maintained in primary culture for up to 6 passages and were plated onto permeable membrane supports not more than 3 hours prior to experimentation. Channel activity was determined by whole cell patch clamp. The standard bath solution was a high NaCl Ringer. The pipette solution was high KCl plus 4mM ATP. Data are expressed as mean ± SEM. Statistical significance was tested using Student’s t-test. Rat brain endothelial cells showed two current types: a time-dependent delayed-rectifying outward current activated by depolarising voltage shifts (more positive than -40mV) and a time-independent inwardly rectifying current active at hyperpolarising voltages, suggesting the expression of Kv-like and Kir-like channels respectively. The outward current was sensitive to 1mM 4AP, reducing peak current recorded at +40mV from 9.9±1.4pApF^-1 to 5.2±1.0pApF^-1 (p<0.001, n=10). The voltage-activated current was inhibited by the Kv1.1 and 1.3 specific blockers dendrotoxin-K (100nM) and margatoxin (10nM) by 55±6 and 57±5 % respectively (p<0.01, n=9/11). The inwardly rectifying current was inhibited by 5mM Ba²⁺. In symmetrical K⁺ solutions, the Kd of Ba²⁺ block of the inwardly rectifying current was found to be of high affinity and voltage sensitive, being 1.0±0.3µM at -120mV and 16.1±4.0µM at -40mV (p<0.05, n=5). This pharmacological profile is typical of a Kir2-type channel and real time PCR analysis identified mRNA for Kir2.1, 2.2 and 2.3 in the rat brain endothelial cells. These data indicate that rat brain endothelial cells express two populations of K⁺ conductance mediated by Kv1.1/1.3 and Kir2 channels respectively. These channels may play an important role in K⁺ recycling necessary in the secretion of brain interstitial fluid.