AbstractResults from this study indicate that caffeine at an embryotoxic dose equal to the LD40administered to 3‐day chick embryos produced both ultrastructural and functional abnormalities in embryonic cardiac mitochondria. The principal effects of caffeine on the ultrastructure of embryonic myocardial cells were clearly suggestive of cellular injury and included: (1) a marked disruption of mitochondrial cristae with formation of intramitochondrial myelin‐like figures and (2) intracellular edema. A biochemical analysis of mitochondrial function revealed that caffeine inhibited the capacity of mitochondria to oxidize succinate. However, when pyruvate and malate were employed as substrates for isolated mitochondria, caffeine did not significantly alter mitochondrial function. Interference with embryonic cardiac mitochondrial succinate oxidation and/or fragmentation of mitochondrial membranes are suggested as possible events in the pathogenesis of caffeine‐induced cardiac cell injury which, in turn, may lead to the embryonic death of the