Few previous studies have targeted how self-determined motivation changes within persons across shorter periods of time (e.g., weeks). Latent growth curve models allow study of within-person change and between-person differences in within-person change over time. The purpose of the study was to study within-person change and between-person differences in change in exercise and motivation in a sample of 2797 exercisers in a natural course of events (i.e. no intervention) over a period of eight weeks. Motivational variables related to self-determination theory were measured by the Basic Psychological Needs in Exercise Scale (BPNES) and the Behavioral Regulation in Exercise Questionnaire-2 (BREQ-2) and Godin Leisure-Time Exercise Questionnaire (GLTEQ) was used to assess self-reported exercise level and intensity. The data was analyzed by latent growth curve models in Mplus. The results show significant increase in the average change (mean slope) of psychological need satisfaction whereas there was significant decrease in amotivation. Furthermore, the slopes of variance were significant for all variables except for autonomy, competence and intrinsic motivation, indicating a pattern of heterogeneity in terms of within-person change. No significant changes were detected in exercise level or intensity (METS). The results will be used as reference data in a future intervention study aiming to enhance self-determined exercise motivation in a comparable population.