Ethyl 8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-4H-imidazo [1,5-a] [1,4]benzodiazepine-3-carboxylate ([11C]RO 15-1788) has been prepared automatically with high specific activity for in vivo visualization or quantitative analysis of brain benzodiazepine receptors. The yield, radiochemical yield, radiochemical purity and specific activity of the product ready for an i.v. injection were 276 +/- 76 mCi, 50.8 +/- 7.8%, 99.3 +/- 0.3% and 2.9 +/- 0.5 Ci/mumol, respectively, taking an average of the latest 3 runs. The time required was about 25 min. Each product was sufficient to carry out three successive clinical studies by positron emission tomography (PET). All the procedures other than evaporation and filtration at the final stage were carried out with specially designed equipment connected to a central control system for radioisotope production.