It is generally recognized that firms face both internal and external pressure to improve their environmental performance. However, few studies have attempted to delineate the importance of those various sources of pressure as firms' managers themselves perceive them. In this study, we show that managers in the Canadian pulp and paper industry perceive government and public, but not financial and consumer markets, as the most important source of pressure. We also show that involvement of the firm's higher level management and environmental education of employees are important determinants of the firm's performance. While the paper provides a better understanding of the determinants of environmental performance, it re-asserts the crucial role of strong government regulatory intervention.