The Direct/Delayed Response Project (DDRP) examines the question 'What is the possible future long-term response of the chemistry of surface waters to continued acidic deposition'. The Geographic Information System (GIS) plays a vital role in a variety of analytical procedures and predictive models. Specific GIS-related tasks within the DDRP includes: the characterization of individual watersheds; the analysis and mapping of regional characteristics; the mapping, display, interpolation and contouring of input/output data; the analysis and mapping of other relevant lake and/or stream chemical data; and the display and interpretation of regional or sub-regional variations in adsorptive characteristics of watersheds. These types of GIS-based analyses contribute significantly to all levels of research within the DDRP. The ability to incorporate and analyze tabular and/or mapped data within the GIS allows researchers at the EPA's Environmental Research Laboratory in Corvallis, Oregon to interactively examine and test various hypotheses concerning the long-term response of surface waters to continued acidic deposition.