The purpose was to examine the effects of an exercise pilot intervention informed by Self-determination theory. The 64 participants were randomized into experimental and control group. The main questions were whether the intervention would influence (a) exercise level, (b) motivation quality, and (c) autonomy and competence need satisfaction. We also examined the indirect effects of self-determined motivation on exercise. Significant intervention effects were found regarding exercise level and motivation quality. Also, intervention effect on exercise was found to be mediated by motivation quality and identified regulation. The results provide interesting information about the underlying mechanisms involved in exercise behaviour change. © Springer Science+Business Media New York 2015