The study investigates the actual genetic development of student teachers' interest in lectures given in a postgraduate language teacher education programme. Students recorded the nature and level of interest at various points of the lecture. The results show that students go through diverse patterns of interest trajectories and that, for the majority, interest development fluctuates between gain and loss. Four types of factors are proposed: triggering, holding, preventing and losing factors. The prospective and retrospective effects of these factors are also examined. For interest to be maintained over a longer period within a two-hour lecture, students must experience a combination of factors. In case of regression, if students demonstrate awareness of the source or solution to the problem, the loss is temporary, tending to recover at the next interval. Some factors, such as use of tasks and interesting specific topics, also help regenerate interest after a loss of interest has occurred. (Contains 4 tables, 1 figure, and 8 notes.)