This study examines the expression of alpha and beta subunits of human chorionic gonadotrophin (HCG) in samples of placental and decidual tissue obtained at 11-15 weeks of gestation from 30 control pregnancies and 11 pregnancies with trisomy 21. In the placental tissue, the concentrations of beta-HCG mRNA and alpha-HCG mRNA were augmented in six and seven of the trisomy 21 cases respectively and in 16 and 14 of the control pregnancies. The median values of beta-HCG mRNA and alpha-HCG mRNA in the two groups were not significantly different. Although the median serum free beta-HCG concentration was significantly (P = 0.03) higher in trisomy 21 pregnancies than the controls, there was no relationship between serum free beta-HCG and relative abundance of beta-HCG mRNA in either the trisomy 21 pregnancies or the controls. Decidual expression of beta-HCG and alpha-HCG mRNA were below detection level in the Northern blot analysis in both the trisomy 21 pregnancies and the controls. These findings suggest that the increase in maternal serum free beta-HCG concentration in trisomy 21 pregnancies occurs during the post-transcriptional phase of HCG protein biosynthesis.