This study empirically examines the temporal nature of students' self-regulatory behavior while learning about a complex science topic using hypermedia. The experiment involved randomly assigning 74 undergraduate students to one of two tutoring conditions: self-regulated learning (SRL) or externally-regulated learning (ERL). Participants in the self-regulated learning condition used hypermedia environment to learn about the circulatory system on their own, while participants in the externally-regulated learning condition also used the hypermedia environment, but were given prompts and feedback from a human tutor during the session to facilitate their self-regulatory behavior. Results from product data (mental model pretest-posttest shifts) indicate that the ERL condition leads to a greater likelihood of having a high-level mental model at posttest. In addition, results from the process (think-aloud) data indicate that having access to a human tutor during learning affects the deployment of certain SRL classes at different times within a learning session. Implications for the design of a computer-based learning environment which is intended to stimulate learners' effective use of self-regulated learning are discussed.