Tacrolimus (FK506) is a potent immunosuppressive drug that, when complexed to a family of immunophilin proteins known as FK binding proteins, inhibits calcineurin in T lymphocytes. Although i.v. use of FK506 in pediatric transplant recipients has been linked to development of cardiomyopathies, its mechanism of cardiotoxicity has not been examined in a neonatal animal model. In our study the effects of FK506 were investigated in cardiac myocytes isolated from newborn piglets. The peak amplitude of electrically triggered fura-2 Ca2+transients was increased in FK506-treated myocytes, but Ca2+transient duration and baseline fura-2 Ca2+ratios were not altered.45Ca2+uptake by digitonin-lysed piglet cells decreased at pCa ≤ 6.0, indicating that sarcoplasmic reticulum efflux channels were leaky. The results suggest that elevated release of sarcoplasmic reticulum Ca2+during systole contributes to cardiotoxic effects observed in children.