We examined how self-regulated learning (SRL) and externally-facilitated self-regulated learning (ERL) differentially affected adolescents' learning about the circulatory system while using hypermedia. A total of 128 middle-school and high school students with little prior knowledge of the topic were randomly assigned to either 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 and declarative knowledge measures) with "process" (think-aloud protocols) data to examine the effectiveness of self- versus externally-facilitated regulated learning. Findings revealed that learners in the ERL condition gained statistically significantly more declarative knowledge and that a greater number of participants in this condition displayed a more advanced mental model on the posttest. 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 used ineffective strategies and engaged in fewer monitoring activities. Based on these findings, we present design principles for adaptive hypermedia learning environments, engineered to foster students' self-regulated learning about complex and challenging science topics.