Individual Day-7 embryos (morulae to expanded blastocysts) were incubated with radiolabelled substrates and karyotyped to determine the sex. In Exp. 1, embryos were incubated for 3 h with D-[1-14C]glucose, as a measure of the activity of the pentose-phosphate pathway (PPP) and D-[5-3H]glucose, as a measure of total glucose metabolism. The labelled products 14CO2 and 3H2O were collected throughout the measurement period. Total glucose metabolism in male embryos was twice that in female embryos and increased between the morula and expanded-blastocyst stages. Relative to total glucose metabolism, PPP activity was four times greater in female than in male embryos. In Exp. 2, embryos were cultured with D-[1-14C]glucose, and L-[3,4-3H(N)]glutamine (a measure of Krebs cycle activity) in the presence of brilliant cresyl blue, a stimulator of the PPP. Glutamine metabolism increased from the morula to expanded-blastocyst stages. Relative to the metabolism of glutamine, the activity of the PPP was one-third greater in female than in male embryos.