Cardiac tissue engineering has a potential to provide functional, synchronously contractile tissue constructs for heart repair, and for studies of development and disease using in vivo-like yet controllable in vitro settings. In both cases, the utilization of bioreactors capable of providing biomimetic culture environments is instrumental for supporting cell differentiation and functional assembly. In the present study, neonatal rat heart cells were cultured on highly porous collagen scaffolds in bioreactors with electrical field stimulation. A hallmark of excitable tissues such as myocardium is the ability to propagate electrical impulses. We utilized the method of optical mapping to measure the electrical impulse propagation. The average conduction velocity recorded for the stimulated constructs (14.4a .1 cm/s) was significantly higher than that of the nonstimulated constructs (8.6 ± 2.3 cm/s, p = 0.003). The measured electrical propagation properties correlated to the contractile behavior and the compositions of tissue constructs. Electrical stimulation during culture significantly improved amplitude of contractions, tissue morphology, and connexin-43 expression compared to the nonsimulated controls. These data provide evidence that electrical stimulation during bioreactor cultivation can improve electrical signal propagation in engineered cardiac constructs. © Copyright 2009, Mary Ann Liebert, Inc. 2009.