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The study’s main objective was to investigate a potential role of pre-conscious processes in exercise behaviour change by comparing the efficiency of two exercise interventions (guided imagery and manipulation of implementation intention).A randomised control trial was carried out with participants allocated to one of four conditions (guided imagery, implementation intention, relaxation, or control).Outcome variables (exercise behaviour, exercise self-efficacy and exercise motivation), were measured pre- and post-intervention. Imagery ability was measured as a potential confounding variable of intervention efficiency. Using provided exercise diaries, fifty sedentary participants (34F, 16M) monitored their physical activity for two weeks. Average of the participants was 29 years (range 19–56).A one-way MANCOVA suggested that 13% of the variation in the linear combination of the dependent variables was accounted for by group allocation. Univariate tests confirmed that significant differences existed between both intervention groups and control group (Implementation Intention − Control (p = 0.048); effect size (d) = 0.92), Guided Imagery − Control (p = 0.012; effect size (d) = 1.46).This study succeeded in increasing exercise behaviour using guided imagery and implementation intention interventions. These are self-managed, economic and practical interventions which may be further developed for particular populations or behaviours. The results may challenge the subjective expected utility basis of social cognition models of health behaviour, as it highlights the importance of considering both conscious and pre-conscious processes antecedent to behaviour change.