This paper presents the results of research into GP perceptions of the impact of on-site counselling on general practice. The research is part of a larger evaluation of a local enhanced primary care mental service. The initial survey and in-depth interviews with GPs reported here focused on the pre-existing counselling service. The results suggest that the benefits of on-site counsellors included reductions in prescribing, cost efficiencies and increased confidence amongst some GPs about providing psychological care themselves. These reported benefits appeared more likely to result when counselling service levels were perceived to be adequate and GPs were satisfied with their partnerships with the counsellors. The key mechanisms to explain the reported benefits were the provision of a safety net for GPs to extend their own practice, particularly when combined with close working relationships with the counsellor. The study has implications for the development of primary care mental health in the UK, particularly in relation to how the interface and working relationships between counsellors and GPs are designed and developed. (Contains 4 tables.)