C ardiac neural crest ablation results in depressed myocardial calcium transients and elevated proliferation in myocardium at a stage when cardiac neural crest cells are not in contact with the myocardium. To test the hypothesis that cardiac neural crest–derived cells, which migrate into the caudal, ventral pharynx at stage 14, block a signal from the ventral pharynx, we cultured stage 12 chick heart tube or myocardial strips in the presence or absence of ventral pharynx. We found that myocardium cultured with ventral pharynx that had not yet contacted neural crest cells had significantly reduced calcium transients and an increased rate of proliferation. Ventral pharynx from intact embryos at a stage when neural crest–derived cells had reached the pharynx had no effect on myocardial calcium transients. Ventral pharynx from neural crest–ablated embryos continued to suppress myocardial calcium transients at this later stage. Myocardium cultured with FGF-2 also showed a significant reduction in calcium transients. An FGF-2–neutralizing Ab reversed the deleterious effect of the ventral pharynx on myocardial calcium transients and proliferation. We therefore examined the expression of FGF-2 and similar FGFs in the ventral pharynx. Only FGF-8 was expressed in a temporospatial pattern that made it a viable candidate for altering the myocardial calcium transient during stages 14–18. In explant cultures, neutralizing Ab for FGF-8 rescued development of the myocardial calcium transient in neural crest–ablated chick embryos.