Primary care trusts (PCTs) will play a major role in commissioning and evaluating new services in primary care. Several studies have discussed the problems associated with conducting evaluative research in primary care generally. However, few GP commissioned services have been formally evaluated, so little is known about the feasibility of such research in the PCT setting. This study uses a tool called force field analysis to identify factors that drive and restrain a primary care-based randomized controlled trial (RCT) of a newly commissioned service for patients with psychosocial problems in primary care, and also to suggest strategies for building on positive factors and reducing negative ones. It involves discussion with general practitioners (GPs) and other primary health care team (PHCT) members in practices with access to the service. Several important dynamics emerge from the analysis with regard to the likely success of evaluation of a new service. These are availability of research skills and funding, prior beliefs about the service and participation in primary care, existing evidence of effectiveness and financial accountability, research design and awareness and ownership in primary care. This study provides evidence that force field analysis can serve as a helpful tool for anticipating and identifying barriers to, and partnerships and actions needed for, successful evaluative research in primary care. It can also facilitate the development of practical strategies to aid successful service evaluation. We suggest that the strategies identified by this study could usefully inform plans for future evaluations in the setting of the PCT.