A floating collagen matrix culture of neonatal rat heart myocardial cells shows rhythmic contractions which are dependent on localization of cells, cell density, and collagen concentration. The rhythmic contractions of the collagen matrix can be registered by a device scanning the optical density at the edge of the gel and have been observed over a temperature range from 9 degrees to 40 degrees C. The results of the present study underline the usefulness of myocardial cell populated collagen matrixes for studies on coherent contractions of heart cell cultures.