The ketone body B-hydroxybutyrate (B-OHB) produces malformations and ultrastructural alterations in mitochondria of mouse embryos exposed for 24 hours to the compound in whole embryo culture. The present study was conducted to establish the time-course of the mitochondrial changes to determine whether the changes are reversible, and to relate these changes to the malformations produced by the compound. Since mitochondria also play a key role in the metabolism of ketone bodies, the capacity of the early somite embryo to metabolize B-OHB was investigated in an effort to link the morphological alterations in the mitochondria to a biochemical process. Early somite embryos were cultured 4, 8, or 24 hours in the presence of 32 mM DL-B-OHB and then cultured for an additional 24 hours in control serum. Finally, embryonic tissue during the teratogenic period was assessed for its capability to oxidize B-OHB using D-(3-14C)-B-OHB. The treated embryos showed progressive alterations in the mitochondria, beginning at 4 hours with a loss of matrix density and culminating at 24 hours with high-amplitude swelling, complete loss of matrix density, and disappearance of cristae. These alterations were reversible following removal of the embryos after 24 hours of exposure to B-OHB and culturing for an additional 24 hours in control serum. Metabolism studies demonstrated that the early somite embryo possesses a limited capacity to oxidatively metabolize B-OHB. The biochemical implications of these findings are discussed with respect to the possible role of ketone bodies in the mechanism of diabetes-induced congenital malformations.