Serum and cerebrospinal fluid (CSF) concentrations of ketoprofen have been measured in 36 patients hospitalised for sciatica. Diagnostic lumbar puncture was done 15 min to 13 h after a single 100 mg intramuscular dose of ketoprofen. Serum and CSF were sampled at the same time. Free serum concentrations were determined by equilibrium dialysis. Total and free concentrations were assayed by a highly sensitive and specific HPLC method. Ketoprofen rapidly crossed the blood-brain barrier and was detected in CSF 15 min after its administration. The rapid diffusion can be explained by the high lipid solubility of the drug. The CSF level was in equilibrium with the free serum concentration from the second to the 13th hours.