A high-performance liquid chromatographic (HPLC) method is described for determining oxytocin, lypressin and other nonapeptides and their by-products in liquid and solid pharmaceutical dosage forms. The use of injection volumes up to 750 microliter permits accurate determination of, e.g., oxytocin in injection solutions containing only 1 I.U. (international unit) per ml. Reproducibilities between 1.0 and 1.5% (relative S.D) have been obtained for liquid dosage forms and up to 3% (relative S.D.) for tablets. The correlation between results obtained by bioassay and by the proposed method is highly significant and suggests the use of HPLC as an alternative technique for stability and content-uniformity tests on dosage forms and concentrates.