The effect of the Xce (x-chromosome controlling element) genotype on the randomness of X-chromosome inactivation in the mouse was studied by monitoring the expression of the X-linked locus pgk-1. The main aim was to test whether the Xce genotype modified the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm. Quantitative electrophoresis of phosphoglycerate kinase (PGK-1) was used to study Pgk-1 expression in the foetus, yolk sac mesoderm and yolk sac endoderm at 13½ days post coitum. The Xcea/Xcec genotype caused non-random X-chromosome expression in the foetus and yolk sac mesoderm. However, there was no evidence that the Xce genotype moderates the preferential expression of the maternally derived X-chromosome in the yolk sac endoderm, as reported by Rastan & Cattanach (1983).