Pain intensity, muscle strength, static muscle endurance, pressure pain threshold, and pain tolerance at tender points and control points were assessed in 31 patients with fibromyalgia (FM), before and after intravenous administration of morphine (9 patients), lidocaine (11 patients), and ketamine (11 patients). The three different studies were double-blind and placebo-controlled. The patients were classified as placebo-responders, responders (decrease in pain intensity by > 50%) and non-responders. The morphine test did not show any significant changes. The lidocaine test showed a pain decrease during and after the infusion. The ketamine test showed a significant reduction in pain intensity during and after the test period. Tenderness at tender points decreased and endurance increased significantly, while muscle strength remained unchanged. The present results support the hypothesis that the NMDA receptors are involved in pain mechanisms in fibromyalgia. These findings also suggest that central sensitization is present in FM and that tender points represent secondary hyperalgesia.