Aims: Recognition of the arrhythmogenic potential of a substance as early as possible is now an essential requirement in the drug discovery and development pipeline. The classical patch-clamp technique is throughput- limited. In contrast the hERG-based assay is limited to the modulation of one ion-channel. Fluorescence-based transmembrane voltage measurements offer a non-invasive and rapid approach to record action potentials on cardiomyocytes. The aim was to adapt, evaluate and improve a potentiometric dye-based platform of the action potential duration and hence the QT-interval evaluation. Methods: The properties of the potentiometric dye di-8-ANEPPS and the genetically encoded sensor "Mermaid" were examined for optical recording of transmembrane voltage and photometry system was adapted accordingly. This method was used to compare adult and neonatal cardiomyocytes upon the effect of several compounds to validate this assay. Results: Quinidine, 4-pyridinamine and E- 4031, which are reported to prolong the cardiac action potentials, were evaluated on neonatal and adult cardiomyocytes respectively, and the efficacies and potencies of those compounds were discriminated clearly by this optical action potential recording approach. The efficacies and potencies of those compounds from this optical imaging are in good agreement with literature data from electrophysiology. Conclusion: Our results show that the adult cardiomyocytes-based optical action potential recording can serve as a base for a reliable assay. This approach provides medium-throughput screening-capability to access early drug cardiac toxicity.