Three mass fragmentatographic methods for the determination of unconjugated estriol in pregnancy plasma and unconjugated and conjugated estriol in plasma of nonpregnant women after estriol administration were developed and tested as to their reliability and practicability. The methods were found to fulfii appropriate reliability criteria especially with regard to specificity. Unconjugated estriol could be assayed without prior chromatography in late pregnancy plasma, but a chromatographic step was needed for plasmas with a low estrioi titre, and both a methylation step and Chromatography is needed to achieve the required specificity if plasma conjugated estriol is determined or if assays are carried out following estrioi administration. Unconjugated estrioi in normal late pregnancy plasma was found in concentrations from 4.3 to 9.3 pg/L The highest value recorded, 16.3 pg/i, was found in a prediabetic subject with mild hypertension, who delivered a child weighing 4150 g by Cesarean section in the 39th week. Low values were found in severe hypertension and in toxemia, and in general the results from the pathological material investigated seemed to correlate well with the clinical findings. However, only a few samples (five to ten) can be processed so rapidly that the resuits can be obtained the same day, which in addition to the expensive and complicated instrumentation limits the usefulness of the methods in routine clinical chemistry.