Measurement of the ventricular action potential (AP) via whole-cell patch clamp is an important contributor to cardiac electrophysiological research. Neonatal rat ventricular cardiomyocytes (NRVCM) are a commonly used model, in particular for stressor- or drug-related questions. High variability of APs and individual methodological settings hinder comparison both in individual studies and, to an even greater degree, between different projects. This study aims to describe sources of AP heterogeneity in NRVCM related to patch clamp measurement with a focus on resolvable causes. Therefore, AP of NRVCM were induced in whole-cell configuration and measured in current-clamp mode. The effects of varying setup temperature, electrode resistance, resting- (RMP), respectively holding membrane potential (HMP), induction approach, current pulse duration and amplitude plus total assay duration were studied and compared to systematically analyzed literature. We analyzed the impact on different output parameters, namely, maximal upstroke velocity (dV/dt), maximal AP amplitude (APA) and AP duration at different percentages (XX%) of repolarization, APD