The effects of transcutaneous electrical nerve stimulation on plasma and cerebrospinal fluid (CSF) levels of beta-Endorphin (beta-EP) were studied in 6 groups of pain-free patients. Different modes of TENS were applied for 20 minutes. Regardless of mode and/or frequency of the stimulation plasma and/or CSF beta-EP levels were significantly increased. Plasma and CSF responses were not correlated suggesting that TENS activates different opiatergic pathways at central and peripheral level.