Transcranial electrical stimulation with high frequency intermittent current (Limoge's) potentiates opiate-induced analgesia: blind studies. Transcutaneous cranial electrical stimulation (TCES) with high frequency (166 kHz) intermittent current (100 Hz: Limoge current) has been used for several years in cardiac, thoracic, abdominal, urological and micro-surgery. The main benefits are a reduced requirement for analgesic drugs, especially opiates, and a long-lasting postoperative analgesia. We have confirmed these clinical observations in rats using the tail-flick latency (TFL) test to measure pain threshold. TCES was not found to modify the pain threshold in drug-free rats, but it potentiated morphine-induced analgesia (systemic injection). To obtain a maximal effect, the stimulation must be initiated 3 h before the drug injection and be maintained throughout the duration of its pharmacological action. TCES potentitation was found to depend on the dose of the drug, the intensity of the current and the polarity of electrodes. These findings were confirmed by blind tests of the efficiency of TCES on several opiate analgesic drugs currently used in human surgery (morphine, fentanyl, alfentanil and dextromoramide). The analgesic effect of these 4 opiates (TFL as % of baseline without or with TCES) were respectively: 174%, 306%; 176%, 336%; 160%, 215%; and 267%, 392%. The results were obtained not only after systemic opiate treatment, but also after intracerebroventricular injection of morphine (10 micrograms; analgesic effect 152%, 207% with TCES) suggesting that TCES potentiation of opiate-induced analgesia is centrally mediated. 