The effects of the "physiologically occurring" D-isomer of beta-hydroxybutyrate (D-BOHB) were evaluated in neurulating mouse embryos by using the technique of whole embryo culture. Following a 24-hour culture period D-BOHB induced malformations and growth retardation in a concentration-dependent manner. At a 48 mM concentration essentially all embryos exhibited neural tube defects, and decreased rates of glucose metabolism by the pentose phosphate pathway (PPP) and Krebs cycle were observed when compared to controls. The relationship between an inhibition of the PPP and induction of malformations by DL-BOHB has been reported, and thereby suggests a similar mechanism may be operating for the D-isomer. In contrast, the effect of the D-isomer on the Krebs cycle may result from a replacement of glucose intermediates by those generated from metabolism of D-BOHB. Concentrations as high as 20 mM D-BOHB have been reported in the serum of uncontrolled diabetic patients, and since ketones rapidly equilibrate across extraembryonic membranes, embryos in vivo may be exposed to concentrations equivalent to those which induced malformations in vitro. However, the incidence of malformations induced by D-BOHB was less than that reported for the DL-racemic mixture at equivalent concentrations, thereby suggesting that the L-isomer is also teratogenic. Therefore, until the presence and concentration of L-BOHB in the serum of diabetics are documented, the assessment of the impact of maternal ketosis on the embryo remains incomplete.