Epidemiological and animal toxicity studies have raised concerns regarding possible adverse health effects of disinfection by-products (DBPs) found in drinking water. The classes and concentrations of DBPs are influenced by the choice of disinfection process (e.g., chlorination, ozonation) as well as source water characteristics (e.g., pH, total organic carbon, bromide content). Disinfected waters were found to contain more than 500 compounds, many of which remain unidentified. Therefore, a "whole-mixture" approach was used to evaluate the toxic potential of alternative disinfection scenarios. An in vivo developmental toxicity screen was used to evaluate the adverse developmental effects of the complex mixtures produced by two different disinfection processes. Water was obtained from East Fork Lake, Ohio; spiked with iodide and bromide; and disinfected either by chlorination or by ozonation/postchlorination, producing finished drinking water suitable for human consumption. These waters were concentrated approximately 130-fold by reverse osmosis membrane techniques. To the extent possible, volatile DBPs lost in the concentration process were spiked back into the concentrates. These concentrates were then provided as drinking water to Sprague-Dawley rats on gestation days 6-16; controls received boiled, distilled, deionized water. The dams (19-20 per group) were allowed to deliver and their litters were examined on postnatal days (PD) 1 and 6. All dams delivered normally, with parturition occurring significantly earlier in the ozonation/postchlorination group. However, no effects on prenatal survival, postnatal survival, or pup weight were evident. Skeletal examination of the PD-6 pups also revealed no treatment effects. Thus, approximately 130-fold higher concentrates of both ozonated/postchlorinated and chlorinated water appeared to exert no adverse developmental effects in this study.