The spatiotemporal distribution of EAP-300 (embryonic avian polypeptide of 300 kDa) was analyzed in embryonic chick heart using immunohistochemistry and confocal microscopy. EAP-300 is a developmentally regulated protein initially characterized in neural cells from chick retina. Myocardial cells all along the early tubular heart were ubiquitously immunolabeled for EAP-300 by embryonic day 2 (E2, Stage 13). At E5 (Stage 24), myocardial EAP-300 expression levels remained significant in both atrial and ventricular myocardium. At E6 (Stage 28), distinct populations of EAP-300 immunolabeled cells were also observed external to the heart, in septal mesenchymal tissue and neural ganglia adjacent to the outflow tract; these cell populations were confirmed as neural crest-derived by co-localization of EAP-300 and HNK-1. At E13 (Stage 39), myocardial immunolabeling for EAP-300 was no longer ubiquitous, but increasingly restricted to conduction tissues, including the atrioventricular bundle and subendocardial Purkinje cells. This restriction of immunolabeling could be demonstrated definitively at E15 (stage 41), by which stage subendocardial and periarterial Purkinje fibers were clearly immunoreactive for EAP-300 and several known markers of chick conduction tissue, including specific myosin heavy chain isoforms and connexin42, a gap junctional protein preferentially expressed by Purkinje fibers. Just prior to hatching at E21 (Stage 46), immunolabeling of conduction tissues was reduced, although still above that of non-conductile myocardium. This spatiotemporal map of cardiac EAP-300 expression indicates that it is independently and transiently expressed in early myocardium, cardiac conduction tissue, and neural crest derivatives during development. ©1995 Wiley-Liss, Inc.