In addition to endogenous natural sex hormones (estrogens), other substances can also exert an estrogenic effect on organisms. Due to the possible impairment of the endocrine system, these substances are classified as endocrine disruptors. In this work, surface water samples from the Baltic Sea and the South China Sea were analyzed for estrogenic substances and estrogenic activity using instrumental analysis and an effect-based bioassay. It was found that both natural and synthetic estrogenic substances are present in both areas and that they are influenced seasonally and regionally.