Abstract Background Randomised trials of knowledge translation strategies for professional behaviour change can provide robust estimates of effectiveness, but offer little insight into the causal mechanisms by which any change is produced. To illustrate the applicability of causal methods within randomised trials, we undertook a theory-based process evaluation study within an implementation trial to explore whether the cognitions of primary care doctors' predicted their test requesting behaviours and, secondly, whether the trial results were mediated by the theoretical constructs. Methods The process evaluation comprised a cross-sectional questionnaire survey of a random 50% sample of the randomised groups of primary care practices in Grampian (NHS Grampian), UK, who took part in a trial of the effect of enhanced feedback and brief educational reminders on test requesting behaviour. The process evaluation was based upon the Theory of Planned Behaviour and focussed on three of the test requesting behaviours that were targeted in the trial -- ferritin, follicle stimulating hormone (FSH), and Helicobacter Pylori serology (HPS). Results The questionnaire was completed by 131 primary care doctors (56%) from 42 (98%) of the sampled practices. Behavioural intention, attitude, and subjective norm were highly correlated for all the tests. There was no evidence that perceived behavioural control was correlated with any of the other measures. Simple linear regression analysis of the rate of test requests on minimum behavioural intentions had R 2 of 11.1%, 12.5%, and 0.1% for ferritin, FSH, and HPS requesting, respectively. Mediational analysis showed that the trial results for ferritin and FSH were partially mediated (between 23% and 78% mediation) through intentions. The HPS trial result was not mediated through intention. Conclusions This study demonstrated that a theory-based process evaluation can provide useful information on causal mechanisms that aid not only interpretation of the trial but also inform future evaluations and intervention development.