Pharmacological inhibition of human-ether-a-go-go-related gene (HERG) K(+) channels by structurally and therapeutically diverse drugs is associated with the 'acquired' form of long QT syndrome and with potentially lethal cardiac arrhythmias. Two aromatic amino-acid residues (Y652 and F656) on the inner (S6) helices are considered to be key constituents of a high affinity drug binding site within the HERG channel pore cavity. Using wild-type (WT) and mutant HERG channels expressed in mammalian cell lines, we have investigated HERG channel current (I(HERG)) blockade at 37+/-1 degrees C by dronedarone (DRONED), a non-iodinated analogue of the Class III antiarrhythmic agent amiodarone (AMIOD). Under our conditions WT I(HERG) tails, measured at -40 mV following activating pulses to +30 mV, were blocked with IC(50) values of approximately 59 and 70 nM for DRONED and AMIOD, respectively. I(HERG) inhibition by DRONED was contingent upon channel gating, with block developing rapidly on membrane depolarization, but with no preference for activated over inactivated channels. High external [K(+)] (94 mM) reduced the potency of I(HERG) inhibition by both DRONED and AMIOD. Strikingly, mutagenesis to alanine of the S6 residue F656 (F656A) failed to eliminate blockade by both DRONED and AMIOD, whilst Y652A had comparatively little effect on DRONED but some effect on AMIOD. These findings demonstrate that high affinity drug blockade of I(HERG) can occur without a strong dependence on the Y652 and F656 aromatic amino-acid residues.