Ca 2+ transients were examined in embryonic chick hearts with an experimentally induced cardiac neural crest-related outflow tract defect known as persistent truncus arteriosus (PTA). In all of the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, suggesting that there is altered development of the myocardium. Earlier reports indicating reduced L-type Ca 2+ current in hearts with PTA suggest that poor viability may be related to impairment of cardiac excitation-contraction coupling. To test this hypothesis, direct measurements of the systolic Ca 2+ transient in fura-2-loaded myocytes from normal hearts and hearts with PTA were carried out. We found that Ca 2-transients were severely depressed in hearts with PTA and difficult to measure above background noise unless signal averaged or treated with isoproterenol (ISO). We confirmed that the reduced Ca 2+ transients were due, at least partly, to a reduction in L-type Ca 2+ current. In addition we found that although ISO could raise the L-type current in hearts with PTA to the level found in normal hearts in the absence of ISO, it could not fully restore the Ca 2+ transient. Furthermore, caffeine-stimulated Ca 2+ transients were diminished in size and the time-to-peak and the decaying phase were significantly slowed. Interestingly, these observations were not accompanied by a reduction in the number of Ca 2+ release channels. These results indicated an impairment of SR function in addition to the reduction in L-type Ca 2+ current. These results strongly support our hypothesis that the poor viability of embryos with PTA is due to impaired cardiac excitation-contraction coupling.