Altered concentrations of brain beta endorphin and spinal cord encephalin have been reported in mice with iatrogenic arachnoiditis. We measured beta-endorphin concentration in cerebrospinal fluid (CSF) in normal monkeys and in monkeys with various degrees of arachnoiditis resulting from myelography and spinal surgery and found a significant negative correlation between the arachnoiditis score and the lumbar CSF beta-endorphin concentration. This study suggests that an assay of CSF beta endorphin can be used to detect the alterations in neurotransmitters in arachnoiditis.