A double-blind, randomized controlled study is designed to assess the analgesic effect of a single intravenous dose of 2 g of magnesium dipyrone (metamizol) and whether the administration of the active drug will be associated with changes in plasma beta-endorphin immunoreactivity values in patients undergoing elective inguinal herniorrhaphy (Bassini operation) under epidural anesthesia. Participants, care givers, and those assessing the outcomes will be blinded to group assignment. Participants will be randomized to receive dipyrone or a placebo. The active drug or placebo will be administered as an intravenous infusion over 10 min. Pain will be evaluated by the patient according to a 100-mm visual analogue scale. Assessments will be carried out the day before surgery, immediately after operation, at the time of drug administration, and 60 and 180 min after treatment. At the same time as pain will be evaluated, blood samples will be drawn for plasma beta-endorphin immunoreactivity measurement (immunoradiometric assay).

Inclusion Criteria:

• 18-70 years, elective surgery, routine laboratory tests (blood cell count, biochemical profile, urinalysis), chest radiography, twelve-lead electrocardiography, body mass index (BMI), blood pressure, pulse rate normal or abnormal values without clinical relevance and a mental status sufficient to be able to complete efficacy tests.

Exclusion Criteria:

• Patients who had taken other analgesics or anti-inflammatory drugs 24 h before surgery, as those with known hypersensitivity to the drug or with any other disorder contradicting the administration of dipyrone. Patients with hypersensitivity to non-steroidal anti-inflammatory agents.

Outcome assessors and data analysts will be kept blinded to the allocation.