Several studies have shown that cardiac tissue slices are a good candidate for drug testing [1-4]. Furthermore cardiac slices as pseudo 2D preparations of heart muscle are a good experimental model to study patho-physiological mechanisms. However, similarities and differences of cardiac slices as compared to whole heart electrophysiological parameters have not yet been investigated in detail. We established a new method to compare action potential (AP) properties of intact whole-heart and cardiac tissue slices from the same heart. Whole Langendorff-perfused hearts from female guinea pigs (300-400g) were bolus loaded with di-4-ANBDQPQ (15μL, 19mg/mL Ethanol). For AP mapping an EMCCD camera (~500fps) was used. A blue LED (CBT90, Luminus) was used [5] to minimize penetration depth, while being efficient to evoke a sufficiently strong signal, weighted towards the subsurface of the left ventricle (LV). The hearts were perfused with modified Krebs buffer containing 10μM blebbistatin. The solution was bubbled with carbogen and recirculated (8mL/min). The heart was held in a mould and the perfusion cannula was fixed to the specimen holder of a microtome. For whole heart stimulation two platinum electrodes where placed about 0.5 cm away from the heart (40V, 2 to 5ms, bipolar) in the bath surrounding the tissue. Several cardiac tissue slices (400μm) were cut tangentially to the LV surface (microtome 7000smz, advance speed 0.02mm/s, blade frequency 80Hz). While the whole heart was imaged in the bath before each section, the cardiac slices were transferred to a special slice imaging station. The bath temperature in both set ups was kept at 35±2oC. The solution of the slice bath was also bubbled with carbogen between measurements, and continuously circulated for additional bubbling. Living tissue slices could successfully obtained from the Langendorff heart, perfused at body temperature, avoiding the need to section from tissue wedges in ice-cold solution. Tissue slices differ from LV tissue in many ways, including mechanical load, electrical source-sink relations, and activation pathways. Slices do deform after sectioning, due to the lack of mechanical strain from neighbouring tissue. Therefore the exact AP pattern in a slice is different from a corresponding tissue area in whole heart. However, slices preserve several key characteristics of the AP, such as overall APD distribution, AP shape, and response to variation in stimulation rate. Therefore, within limits, cardiac tissue slices may be a useful model for exploration of cardiac electrophysiology in relation to identifiable underlying tissue sources, and may be particularly useful for differential studies involving before/after investigations, such as for the study of drug effects.