Introduction The aim of our study was to assess the changes of free radicals and other biochemical parameters after nociceptive stimulation in patients. We detected whether preemptive analgesia had a positive effect on acute postoperative pain. Patients and methods:diagnosis: hernia inquinalis,operation:herniotomia,group of patients (n=63), men (n=50), women (n=13),timing of the investigations from blood samples: immediatelly before operation ,4 hours after operation ,24 hours after operation. Patients were divided into three groups: without premedication, the premedication with pethidin,the premedication with morphine .We measured biochemical parametres: proteins, albumins, prealbumins, apolipoproteins A, B, CRP, glucose, lipids (LDL, HDL, total cholesterol, triacylglycerols) and uric acid. Results We found out that nitroxid and hydroxyl free radicals and singlet oxygen increased significantly after nociceptive stimulation. In particular, we measured the free hydroxyl radicals and singlet oxygen by EPR method. Our goal was to find the most effective combination for the effective analgesic treatment of acute postoperative pain. Free radicals increased in individual samples during the postoperative course in pethidine and without premedication. After application of morphine the free radicals were insignificantly reduced,especially in the third sampling. The increase in measured values of free radicals is statistically significant in patients without premedication.Beside finding an optimal preemptive and postoperative analgesia, we tried to understand the mechanisms of postoperative pain and oxidative stress. We were able to objectify the biochemical changes in acute postoperative pain on the basis of postoperative stress. This method is absolutely a priority and has never been used before. Earlier experimental results were partially clinically verified using different types of acute and chronic pain in humans. In all these pain syndromes are different biochemical changes. Our next goal was to find the most effective combination for the effective analgesic treatment of acute postoperative pain. Conclusions We studied postoperative analgesia and the effects of various types of analgetics, including preoperative and postoperative medication and its objective evaluation. We focused on an objective assessment of pain using different biochemical parameters, especially the free radicals that are significantly changed during acute pain. It was shown that morphine is the best indication in preemptive analgesia prior to surgery with an expected moderate pain. Morphine in comparison with Pethidine and the group without premedication only significantly reduced levels of free radicals 24 hours after surgery. We tried to explain the relationship of postoperative pain and oxidative stress.