This report describes for the first time a novel anionic background current (I-AB) identified in guinea-pig isolated ventricular myocytes. It also shows that IB has both novel and differential pharmacology from other (cardiac) chloride currents. Using the whole-cell patch-clamp technique and external anion substitution, I-AB was found to be outwardly rectifying and highly permeable to NO 3, with a relative permeability sequence of NO3- > I- > Cl-. I-AB was not blocked by 50 muM DIDS, by hypertonic external solution, or by the nonselective protein kinase inhibitor H7-DHC. Exposure to the pyrethroid agent tefluthrin (10 muM) increased the current density of IAB significantly at positive voltages (P < 0.05), but had no significant effect on other cardiac chloride currents. We conclude that I-AB possesses a distinct pharmacology and does not fall into the three major classes of cardiac chloride conductance commonly reported. (C) 2002 Elsevier Science (USA).