The primary metabolic functions of lipogenesis and lipolysis in white fat adipocytes are regulated by catecholamines and insulin. These agents have been shown known to affect the resting membrane potential of adipocytes. Nonetheless, the electrophysiology of white adipocytes is poorly studied. Non-selective cation channels and voltage-dependent K+ channel have been identified in rat white adipocytes. With regards to anion-selective (Cl-) channels, these have been described in brown adipocytes (Pappone and Lee, 1995), but their presence in white adipose cells has not been previously reported. Thus, we have used the cell-attached patch-clamp technique to explore the existence of Cl- channels in differentiated 3T3-L1 adipocytes, an established experimental model of white fat adipocytes. With Cl- as the major anion in the pipette solution, multiple amplitude single-channel currents were to be found to be spontaneously active in 80 out of 86 patches. Currents were outward at pipette potentials positive to -20 mV and had an ohmic relationship with holding potential. In normal high K+ pipette solution, the median single-channel slope conductance was 13.8+/-0.8 pS (mean +/- S.E.M, n=8), similar slope conductances were observed in pipette solutions where the major cation was either Na+, TEA+, NMG+, or K+ (with or without Mg2+ or Ca2+) with Cl- as the anion. On the contrary, when Cl- in the pipette solution was substituted for aspartate, the occurrence of single-channel currents was significantly reduced to only 4 in 20 patches (χ2 p<0.0001). Under these conditions the median single-channel conductance was 3.9+/-1.9 pS (n=4). In conclusion, the major ion channel isoform active in the plasma membrane of 3T3-L1 adipocytes appears to be Cl- selective; further studies are needed to determine its existence in rat adipocytes and also clarify its function and regulation.