The purpose of this study was to determine possible genotoxic and cytotoxic (or mitogenic) effects of high concentrations of oxytocin, active component of Syntocinon in cultures of human peripheral blood lymphocytes. Two test systems were used: (1) analysis of numerical and structural chromosome aberrations, and (2) the in vitro sister chromatid exchange (SCE) test. On the basis of the results obtained it can be concluded that oxytocin does not express any genotoxical properties. Furthermore, the mitotic index did not change significantly.