Uncertainty in the input parameters used in simulation studies needs to be taken into account when evaluating the results produced. This work considers the effect of using various sets of bidomain conductivity values in two simulations: firstly, activation times, which indicate the propagation of the depolarisation wavefront through the cardiac tissue, and, secondly, the determination of defibrillation thresholds in a ‘heart in a bath’ model, where a shock is delivered from two opposing patch electrodes. Both simulations use the same two types of sets of bidomain conductivity values: four-conductivity datasets (where normal and transverse conductivities are assumed equal) and sixconductivity datasets, including newly proposed sets that are based on experimental measurements. The activation time maps show significant differences depending on the conductivity set used, as do the defibrillation thresholds. The defibrillation thresholds vary by more than 20%, whereas the activation times for epicardial breakthrough and total depolarisation both vary by approximately 50%. It is found that the most extreme values in each case are produced by two of the four-conductivity datasets. Since these differences are large enough to lead to different conclusions in such studies, it is suggested that the four-conductivity datasets may not be an appropriate choice for use in simulation studies in the heart.