By the second day in culture, the heart cells are adherent in monolayer clumps to the cover slip and each clump is beating spontaneously, rhythmically and simultaneously just as in the intact heart. A cover slip with cells is transferred to a temperature controlled perfusion chamber on an inverted microscope. With a video camera, a monitor screen, and an edge-monitor one can focus on the contraction of a single myocyte in a clump of myocytes and with a recorder preserve a tracing of the contraction amplitude and rate of contractions.