The effect of alterations in ketone body availability during the early postnatal period on the in vitro and in vivo utilization of beta-hydroxybutyrate (beta OHB) by developing rat brain was determined. Ketone body availability was prolonged by feeding a high fat diet during pregnancy and lactation and to pups after weaning. Availability was decreased by early weaning pups at 16 days to a low fat liquid diet. In the in vitro studies, oxidation of [14C]beta OHB to CO2 and incorporation into brain lipid was determined at 7, 14, 21, and 35 days of age. In the in vivo studies incorporation of [14C]beta OHB and [3H]glucose into brain lipid was measured. The results indicate that it is possible to alter the brain's utilization of beta OHB for lipid synthesis and energy metabolism during the developmental period. This was particularly evident in the in vivo studies. The in vivo results also suggest a relationship between beta OHB availability in the blood and incorporation into brain lipid. When availability was decreased by early weaning pups to the low fat diet, incorporation of beta OHB into lipid was also decreased. Likewise, when availability was increased, such as was seen in the high fat feeding study, incorporation was increased.