Optogenetics is an emerging technology for the manipulation and control of 
excitable tissues, such as the brain and heart. As this technique requires the 
genetic modification of cells in order to inscribe light sensitivity, for 
cardiac applications, here we describe the process through which neonatal rat 
ventricular myocytes are virally infected in vitro with channelrhodopsin-2 
(ChR2). We also describe in detail the procedure for quantitatively determining 
the optimal viral dosage, including instructions for patterning gene expression 
in multicellular cardiomyocyte preparations (cardiac syncytia) to simulate 
potential in vivo transgene distributions. Finally, we address optical actuation 
of ChR2-transduced cells and means to measure their functional response to 
light.