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