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Recent studies in the Douro River estuary show signs of pollution in the area and of fish endocrine disruption. However, the chemical nature of the local contamination has not been fully investigated nor have studies checking for the simultaneous presence of endocrine-disrupting chemicals (EDCs), either of animal (estrone, E1; estradiol, E2), pharmaceutical (17 alpha-ethynylestradiol, EE2), vegetal (daidzein, DAID; genistein, GEN; biochanin A, BIO-A), or industrial (bisphenol A, BPA; 4-octylphenol, 4-OP; 4-nonylphenol, 4-NP) origins. Thus, the main objective of this study was to examine the presence of these EDCs in estuarine water samples collected, in every season of the year, at nine sampling stations along the estuarine gradient. All samples were processed by two-step solid-phase extraction (Oasis HLB followed by silica) prior to high-performance liquid chromatography with diode-array detection (HPLC-DAD) and gas chromatography-mass spectrometry (GC-MS) analyses. The current data showed that E1 and EE2, all phytoestrogens, and BPA were identified and measured in this estuary. In contrast, 4-OP was only detected by GC-MS and E2 and 4-NP were not found. Additionally, E1 (up to 112.9 ng/L) and EE2 (up to 101.9 ng/L) were both measured in biologically hazardous amounts in winter. In the year sampled, the phytoestrogens suggested a possible seasonal pattern of fluctuation. Both DAID (up to 888.4 ng/L) and GEN (183.6 ng/L) were maximal in early summer, whereas BIO-A (up to 191.4 ng/L) reached its highest concentrations in winter. BPA (up to 10.7 mu g/L) also attained highest levels in winter. In December 2005, it is hypothesized that E1, EE2, and BPA concentrations were atypically high due to current drought conditions. Almost all assayed EDCs existed in all seasons and, therefore, might have contributed to endocrine disruption of aquatic animals, previously documented by the high rate of ovotestis in fish caught in this estuary.