Based on the important relationship of calcitonin and serotonin with narcotic drugs at the level of the central nervous system and with the modulating mechanisms of pain sensation, we have studied whether central baseline levels of these modulators possess a predictive value for the assessment of the intensity and duration of narcotic spinal analgesia. Therefore, intradural levels of calcitonin an serotonin were measured in 18 patients who underwent and infraumbilical laparotomy with intradural anesthesia with bupivacaine associated with 0.5 mg of morphine. There was no statistically significant correlation between the levels of calcitonin and serotonin with the age, weight, height, sex, values of analogic visual scale 2, 4, 6, 12 and 24 hours after lumbar punction, and duration of the analgesia. Even though a correlation exists in experimental models, our results indicate that baseline values of modulators calcitonin and serotonin lack of predictive value for the assessment of the degree and duration of narcotic spinal analgesia.