A simulated distribution system (SDS) method has been developed to predict the amounts of disinfection by-products (DBPs) that would form in a distribution system. The parameters (chlorine dosage, incubation temperature, and incubation holding time) of the method are chosen to simulate the conditions of the treatment plant and the distribution system. A study was conducted on the Metropolitan Water District of Southern California's distribution system at a time when free chlorine was being used in the distribution system. A second study was conducted on water from a utility in the eastern United States. Both studies showed good correlation between the SDS samples and the samples collected from the distribution systems. Includes 15 references, tables, figures.