BACKGROUND: Protected learning time (PLT) schemes have been set up in primary care across the UK. There is little published evidence of their effectiveness. OBJECTIVE: To investigate the effect of a PLT intervention for general practice to increase prescribing of ramipril for prevention of cardiovascular outcomes. DESIGN: Quasi-experimental, interrupted time series. SETTING: Lincolnshire, UK. METHODS: Prescribing data were analysed one year before and after the education for change in rate of increase of prescribing of ramipril, whether change in prescribing was related to postulated explanatory variables and to determine intervention costs. MAIN OUTCOME: The primary outcome was the rate of change of ramipril (10 mg) prescription items 12 months after compared with before the educational intervention. Secondary outcomes included cost. RESULTS: Ramipril prescribing at therapeutic dosage increased significantly (odds ratio 1.50, 95% CI 1.07-1.93) following education by 52,345 items (31,132 items at 10 mg) at a cost of pound 292k to pound 460k depending on formulation. This occurred despite a background of secular change. Most practices were represented by GPs, nurses or both during the education. Single-handed GPs were less likely to attend. Practices showed considerable variation in response to the educational intervention. The only predictor of whether practices increased in prescribing rate after the education was whether a practice nurse had undertaken specific diabetes training. Total list size, dispensing, training or single-handed status and GP attendance did not predict a change in prescribing. CONCLUSION: PLT schemes can contribute to beneficial changes in prescribing across a large geographical area.