The objective of this study was to measure the rate of formation of two classes of disinfection byproducts (DBPs) in a simulted pipe environment and compare it with rates observed for the same water held in glass bottles. The DBPs studied were trihalomethanes and haloacetic acids, both of which are currently regulated by the US Environmental Protection Agency under its Disinfectants/Disinfection Byproducts Rule. The simulated pipe environment was designed to replicate actual flow conditions within a ductile iron pipe that had been subject to significant corrosion and biofilm buildup. The pipe simulator used in this study was an 88-foot (27-meter) long loop of 6-inch (150mm) diameter, unlined ductile iron pipe equipped with a recirculation pump and a heat exchanger cooling system. The pipe had been inservice for several years and showed evidence of considerable uniform corrosion and tubercle buildup on its inside surface. Examination of coupons placed flush with the pipe wall also confirmed the presence of a significant amount of biofilm. Two 300-gallon feed tanks, connected in parallel were used to feed test water to the simulator. A pair of cartridge filters (1.0 and 0.5 micron pore size) were placed in series on the inlet line between the test water reservoir and the feed tanks. Includes 17 references, tables, figures.