Abstract: Estriol (E 3) was qualitatively and quantitatively determined by GC/MSD with an internal standard, 3-cyclopentanoxy-estrone (CE 1). Urine sample was hydrolyzed with hydrochloric aid and then extracted with ether. Finally it was derived with acetic anhydride and pyridine. The method was used in determination of the urine of 2 patients who were suffered from radiation accidents and then treated with E 3. The results showed that E 3(OAc) 3 was excreted as its prototype and the excretion was mainly excreted during first three days.