After dinner at 6 P.M. food was withheld from non-pregnant women and women in the third trimester of pregnancy: there were lean and obese women in each group, and all had normal carbohydrate metabolism. Levels of circulating fuels and glucoregulatory hormones were monitored between 6 A.M. (12 h fast) and noon (18 h fast) the following day. In non-pregnant women plasma levels of glucose, alanine, free fatty acids (FFA) beta-hydroxybutyrate, insulin, and glucagon remained constant between 6 A.M. and noon except for slight fall in plasma alanine in the obese women and slight increases in beta-hydroxybutyrate in the lean. By contrast, in pregnant women the concentrations of all substances measured changed profoundly after extension of the overnight fast. Levels of plasma glucose, which were significantly lower than those in non-pregnant women at 6 A.M., fell further during the subsequent 6 h; plasma alanine levels declined significantly during the same interval, and there were coincident rises in plasma FFA and beta-hydroxybutyrate and reductions in plasma insulin. The changes in lean and obese women were of similar magnitude; habitus did not therefore seem to have effects additional to those of pregnancy on the response to skipping breakfast. In all the pregnant women final levels of FFA and beta-hydroxybutyrate were strongly correlated and inversely related to final levels of plasma glucose. The studies indicate that "accelerated starvation" can be unmasked during pregnancy even with the minor dietary deprivation common in conventional clinical circumstances. Thus, insofar as it may be desirable to avoid ketonaemia during pregnancy, the common practice of skipping breakfast whether by personal preference or for doctor-initiated laboratory testing should be avoided in pregnant women.