We have conducted a comprehensive 4-year study of potential endocrine-disrupting organic chemical levels in four northern and southern Missouri streams that received runoff from confined hogs, pastured cattle and row crops. From each location, monthly collection of 24-hour composite water samples were analyzed using solid phase extraction (SPE) and gas chromatography-mass spectrometry (GC-MS) methods for the identification and quantification of target organic contaminants. Most commonly detected chemicals were metolachlor, atrazine, nonylphenol, bisphenol-A, alkyl phthalates, etc. There was a direct relationship between the observed water pollution by pesticide/herbicide chemicals and the agricultural practices of the area, as regards to seasonal application.