Abstract: The invention relates to an automatic chemical injection system for injecting metered amounts of chemical into flush water as the flush water enters a toilet. The injector assembly is easily mounted into the flush water supply line. The assembly has an injector housing which has a truncated cone-shaped bore therein through which the flush water passes. The truncated cone-shaped bore has a smaller diameter opening which increases to a larger diameter opening in the direction of water flow. A smaller bore intersects the cone-shape bore adjacent the smaller diameter opening so that a venturi effect occurs at the opening of the smaller bore. The smaller bore is interconnected to a chemical reservoir so that chemical is pulled into the flush water as the flush water passes the smaller bore. A check valve is located in the smaller bore to permit chemical to be metered into the flush water but not permit flush water to back up into the chemical reservoir and dilute the chemical reservoir.