OBJECTIVE The aim was to investigate whether the use dependent effects of antiarrhythmic drugs on the Na+ current could be applied to explain their effects on impulse conduction. METHODS Trains of rapid stimuli were applied to guinea pig papillary muscles via an electrode in the presence of quinidine (20 and 60 mumol.litre-1), and the conduction velocity was determined from the time difference between two signals of the maximal rate of rise (dV/dtmax) of the action potentials at two separate sites. The relationship of the time constants of the onset and recovery from the use dependent inhibition induced by quinidine was determined for the dV/dtmax and the conduction velocity. METHODS Six male Hartley guinea pigs weighing 200 to 300 g were killed by a blow to the head and the papillary muscles were rapidly excised from the right ventricles. The preparations were superfused with Tyrode solution. RESULTS The rate of onset of the use dependent inhibition of conduction velocity and that of the square of conduction velocity were both faster than the simultaneously measured rate of onset of dV/dtmax inhibition induced by 20 mumol.litre-1 quinidine at high frequency stimulation. The relation between the rates of onset of the use dependent inhibition of conduction velocity (and the square of conduction velocity) and dV/dtmax became weak with low frequency stimulation and in the presence of 60 mumol.litre-1 quinidine. However, the recovery of conduction velocity (and the square of conduction velocity) from quinidine induced use dependent blockade, as measured by the extrastimulation method, appeared to be slower than the recovery of dV/dtmax. These results may be explained by a transient change in intracellular and intercellular conditions, such as an increase in internal resistance. CONCLUSIONS The onset and recovery of the use dependent inhibition of conduction by antiarrhythmic drug may not always parallel the changes of the dV/dtmax of action potential in multicellular muscle preparations.