This report summarizes the activities of the Contaminants Component of the Northern River Basins Study to address the following questions: How the aquatic ecosystems in the Peace, Athabasca, and Slave River basins been affected by exposure to toxic compounds; and what long-term monitoring programs and predictive models are required to provide ongoing assessment of the state of those ecosystems. Research is described in four project areas: A basin-wide survey of biochemical responses to organochlorines and other contaminants in major fish species; a basin-wide survey of the toxicity in bottom and suspended sediments; an assessment of the utility of semi-permeable membrane devices as potential substitutes for wild fish in a long-term monitoring program; and assessment of the feasibility of using small, locally resident fish species as alternates to large adult fish in a long-term biological effects monitoring program.