In this research, two large scale MARKAL L. P. models of Québec and Ontario are used to evaluate the response of their energy systems to various economic and environmental scenarios. Two contrasted economic scenarios, TREND and GREEN, are combined with five levels of CO₂ tax, to create ten combinations. The effects of each combination of systems costs as well as the energy and technology substitutions are analyzed. The most promising responses appear to be conservation, and switching away from fossil fuels and toward hydro and nuclear electricity, and toward fuels derived from biomass.