Because of the importance of blood flow from the mother to fetus, and the role of the main uterine artery to convey blood. The effect of ergotamine as a vasoconstrictor on the contractility of such artery was studied. Thus, light was shed on the importance of the lining epithelium in counter acting the effect of some vasoconstrictors and the role of EDRF (Endothelium Derived Releasing Factor). The main uterine artery was isolated and removed without stretching, placed in a modified Krebs-Henseleit solution and oxygenated with carbogen. Some pieces were denuded and the others were kept intact ones. After equilibration, 5- hydroxytryp-Lamine (5-HT) was added to the bath of all tissues and dose response curve (DRC) for 5-HT was constructed. Ergotamine was added in different concentrations: 10^-6, 10^-7 and 10^-8 to all tissues over 1 hour. Then DRC of 5-HT is constructed. Ergotamine diminished 52% of the contractile effect of 5-HT. The present work indicates that the endothelium of the main uterine artery in Pegnant ewes near term play a role in keeping the contractile power of uterine artery less responsive to vasoconstrictors like ergotamine in pregnant ewes.