Paired specimens of cerebrospinal fluid and serum were taken from 21 patients to estimate penicillin concentrations two to three hours after the last dose of a course of 14-21 daily intramuscular injections of procaine penicillin 0.6 MU. Of 10 patients treated with procaine penicillin alone, eight had no detectable penicillin and two had sub-treponemicidal concentrations (less than 0.018 mg/l) in the cerebrospinal fluid. Of 11 patients treated with procaine penicillin as above and probenecid 2 g a day by mouth, three had no detectable penicillin, two had sub-treponemicidal concentrations, and six had treponemicidal concentrations of penicillin in the cerebrospinal fluid. All 21 patients had treponemicidal concentrations of penicillin in the serum. This dose of procaine penicillin alone or with probenecid is therefore not recommended for treating neurosyphilis.