Based on predictions drawn from self-determination theory (SDT: Deci & Ryan, 2000, The “what” and the “why” of goal pursuits: Human needs and the self-determination of behavior. Psychological Inquiry, 11, 227–268) this study examined specific differential mediating effects of psychological need satisfaction in the relation between support for psychological needs and the internalization of behavioural regulation for exercise.133 former female exercise referral scheme clients (mean age = 54.51) completed measures of need support provided by their exercise practitioners, satisfaction of the psychological needs for autonomy, competence and relatedness, the latter including measures of interpersonal relatedness and social assimilation, and behavioural regulations for exercise.Multiple mediator regression analyses showed that when need support promoted autonomy and social assimilation, individuals were less amotivated and less externally regulated. Fostering personal relatedness whilst not fostering autonomy was associated with greater introjected regulation but promoting social assimilation served to partially offset this negative effect. When need support facilitated autonomy, competence and personal relatedness, identified regulation was promoted. Satisfaction of autonomy and competence needs mediated the association between need support and intrinsic regulation.The results support the central role afforded to autonomy in SDT and indicate that autonomy does not have to be actively undermined in order to forestall the internalization process. In practical terms, in addition to promoting autonomy and competence, exercise practitioners should help referral schemes clients to assimilate into the social environment of exercise facilities as well as ensuring that they receive more direct interpersonal support.