Two sources of toxic chemicals are readily recognized in recycling wastewater for potable purposes: Those introduced by the prior use of the water and those introduced during treatment and distribution of the water. Historically, the focus in water treatment has been directed towards the former source and with little attention being paid to the latter. In a system with a source that is made up of primarily domestic types of waste, the production of by-products with treatment chemicals and the leaching of materials from surfaces that contact potable water are important sources of toxic chemicals. The most instructive example involves by-products generated during disinfection. A system such as that contemplated for the space station may well magnify these problems unless the treatment is designed to prevent chemicals from these sources from accumulating in the water system. The present paper focuses on the genesis of these problems and the toxicological hazards that result.