Sphingolipid metabolism is disrupted in Cert^gt/gt embryos. The total cell and plasma membrane sphingomyelins and ceramides were estimated in E10.5 Cert^+/+ (+/+) and Cert^gt/gt (−/−) embryos as described under Materials and methods. (A) The total sphingomyelin levels are decreased in mutant embryos. (B) The total cellular ceramides are indistinguishable between the mutant and the wild type. (C) The plasma membrane ceramides are decreased in the mutant embryos. (D) CERT deficiency and lack of ceramide transfer out of the ER cause increased ceramide levels by 200% in the ER. (E) The plasma membrane sphingomyelins are decreased by 52% in the CERT mutant embryos. (F) Plasma membranes were prepared by density gradient centrifugation using Opti-prep, and membrane fluidity was measured. No significant differences were found in the fluidity of plasma membrane between the wild type and the mutant. Error bars indicate standard deviation.