Levels of disinfection by-products (DBPs) in drinking water samples tended to be higher in summer than in winter. The concentrations of some DBPs tended to increase with distance (time) from the treatment plant while other DBPs increased in the ยฎrst part of the distribution system and then decreased at a further distance from the treatment plant. Samples taken near the end of the distribution system provided an estimation of maximum exposure for trihalomethanes but not for haloacetic acids. Maximum values for DCAA and TCAA usually occurred at some point within the distribution system and by the end of the distribution system DCAA and TCAA levels were below their respective maximum values. No single sampling location or season provided simultaneous maximum values for the trihalomethanes and haloacetic acids. The diculties involved in determining current human exposures to DBPs in drinking water emphasize the problems involved in attempting a retrospective estimation of DBP exposure. It will be dicult to assess the risk of adverse health eects within an exposed population when the levels and speciation of the DBPs change between seasons and the level of exposure can depend on how far the consumer lives from the water treatment plant.