This video shows the changes in optical thickness (OT) of spontaneously beating rat cardiac myocytes as imaged with the BioCam Quantitative Phase Microscope. The myocytes were measured in a Bioptechs FCS3 perfusion chamber with an aluminized base plate and a 100μm gasket. Cells were kept at 37°C with an HBSS (Hank’s balanced salt solution) fluid bath. The video on the left shows a 2D representation of the OT data, while the video at right shows the data in 3D. The maps were acquired at 15fps.