This study examined the role of different goal setting instructional interventions in facilitating high school students' regulation of their conceptual understanding of ecological systems while using a Web-based water quality simulation environment. Building on the information processing theory of self-regulated learning (SRL) of P. Winne and colleagues, the study examined: (1) students' self-regulation; (2) co-regulation; and (3) the role of the teacher as an external regulator during a knowledge construction activity. Sixteen 11th and 12th graders were randomly assigned to one of two goal-setting instructional conditions (teacher-set goals (TSG)and learner generated sub-goals (LGSG)). Students used RiverWeb (tm), the simulation, collaboratively during a 3-week curriculum on environmental science. The students' emerging understanding was assessed using their pretest and posttest scores, and was also assessed through an analysis of their discourse during several collaborative problem-solving episodes. The LGSG condition facilitated a shift in students' mental models significantly more than did the TSG condition. Students in the LGSG condition were also better at regulating and co-regulating their learning ability during the knowledge construction activity. In general, they planned and monitored their learning more efficiently by creating sub-goals, activating prior knowledge, and engaging in adaptive help-seeking. They also used more effective learning strategies and were more effective in handling task difficulties and demands than was the TSG group. Results provide an initial characterization of the complexity of self- and co-regulated learning in a complex, dynamic technology-enhanced student-centered classroom. (Contains 1 table, 5 figures, and 49 references.) (Author/SLD)