The analgesic action of tramadol in the post-operative period was compared with that of an analgesic combination (Nisidin) in a sample of 60 patients (31 male, 29 female) aged between 20 and 70 years undergoing surgical operations on the abdomen involving opening of the peritoneum. The study was carried out according to a controlled and randomized experimental design. Tramadol in 100 mg/2 ml vials and Nisidin in 2 ml vials were administered parenterally at a dosage of three vials a day during the first three days of the post-operative period. The analgesic action of tramadol was shown to be more effective than that of Nisidin both after the first dose and during the three days of observation (p < 0.01). Local and general safety were good in both groups, and respiratory and heart rates did not undergo clinically significant modifications. In conclusion, tramadol proves to be particularly indicated in the treatment of post-operative pain, given that it has analgesic action combined with good local and general safety.