OBJECTIVE To investigate the mechanism by which cocaethylene, a metabolite of cocaine and alcohol inhibits a cardiac delayed rectifier potassium channel. METHODS The cDNA of the HERG potassium channel that underlies I(Kr) in humans was transiently expressed in tsA201 cells and currents recorded using the patch clamp technique. RESULTS The cocaethylene inhibition of HERG is concentration-dependent with an IC(50) of 4.0 microM. The inhibition increases over the range of voltages where the channels activate suggesting that cocaethylene binding may be linked to the activation or opening of the channels. Cocaethylene slows the deactivation of the tail current indicating that drug-modified channels are stabilized in the open conformation. Cocaethylene also accelerates inactivation but has no effect of the recovery from inactivation. CONCLUSIONS Cocaethylene inhibits HERG by binding to the activated or open channels and by modulating the kinetics of inactivation. The cocaethylene inhibition of the channels occurs within the range of concentrations detected in the plasma of humans following the ingestion of cocaine and alcohol and is likely to contribute to the potent cardiotoxicity of this drug combination.