Information on the morphology of mitochondria during embryogenesis is scattered in the literature, but there appears to be a consistent pattern. During early organogenesis, the embryo is in a state of relative hypoxia associated with a major decrease in terminal electron transport system activity and a marked increase in anaerobic glycolysis. Ultrastructural studies of a 14-somite monkey embryo and day 10 and 12 rat embryos, together with a review of the literature, led us to determine that this hypoxic stage is characterized by vesiculation of the mitochondrial inner membranes, or cristae. Starting in the late morula stage and continuing during early postimplantation embryogenesis, the cristae increase but appear tubular or vesicular. After the end of neurulation, and with the onset of vascular perfusion of embryonic tissues, the cristae gradually become lamellated; by the limb bud stage they appear more mature. We suggest that new cristae derive from blebs of the inner mitochondrial membrane and that with maturation these blebs collapse, giving them a lamelliform appearance. The delamellated state of the cristae might inactivate oxidative phosphorylation to protect the embryo from toxic respiratory end-products that could accumulate in an embryo before there is vascular perfusion. Consistent with this hypothesis, mitochondrial diameters in the developing heart of monkey and rat embryos were approximately twice those found in skin and neural tube.