In this study we examined the effectiveness of self-regulated learning (SRL) and externally-regulated learning (ERL) on adolescents' learning about the circulatory system with hypermedia. A total of 128 middle-school and high school students with little knowledge of the topic were randomly assigned either to the SRL or ERL condition. Learners in the SRL condition regulated their own learning, while learners in the ERL condition had access to a human tutor who facilitated their self-regulated learning. We converged product (pretest-posttest shifts in students' mental models) with process (think-aloud) data to examine the effectiveness of self- and externally-regulated learning about a science topic during a 40-minute session. Findings revealed that the ERL condition facilitated the shift in learners' mental models significantly more than did the SRL condition. Verbal protocol data indicated that learners in the ERL condition regulated their learning by activating prior knowledge, engaging in several monitoring activities, deploying several effective strategies, and engaging in adaptive help-seeking. By contrast, learners in the SRL condition regulated their learning by using fewer monitoring activities, and using several ineffective strategies. We present design principles for adaptive hypermedia learning environments designed to foster students' self-regulated learning of complex and challenging science topics.