In the first part of this report a methodology is described which allows an objective and specific exploration of experimental pain in man by using some electrophysiological features of cutaneous reflexes. This method can be summarized as follows: in normal and trained volunteers, we studied simultaneously the recruitment curves of the nociceptive flexion reflex of a knee-flexor muscle (biceps femoris muscle) and that of pain sensation elicited by electrical stimulation of the ipsilateral sural nerve at the ankle. In this procedure, we found that the reflex threshold (Tr) was closely related to that of pain threshold (Tp) around a similar value (10 mA). In the same way, both the threshold of the maximal recruitment of the reflex (Tmr) and that of tolerance to pain (Tpt) were found to be close to 33 mA. These values are reliably reproducible in one subject from one session to the other, and in all subjects with minimal inter-individual variations and without any significant inter-sex difference. These close relationships between Tr and Tp and between Tmr and Tpt respectively constituted the basic ground for the elaboration of the methodology for investigating objectively the human nociceptive reactions. In the second part of the paper, this methodology is applied for studying the spinal mechanisms of morphine analgesia when the drug is given either by intravenous route in normal subjects and in paraplegic patients or administered epidurally in patients with acute postoperative pain. On the one hand, the resulting data strongly validate the model since they show that pain and nociceptive reflex are similarly depressed by morphine in a dose-response fashion. On the other hand, data also show that the spinal level is one of the main important sites of the mechanisms of morphine-induced analgesia since this drug is found to strongly depress selectively the nociceptive transmission directly at the spinal level. Finally, this method is applied for investigating the nociceptive reactions in patients affected either with a pathological lack of pain sensation or, by contrast, in patients complaining of acute or chronic pain from various origins. Since the nociceptive flexion reflex can be considered as a specific and objective physiological correlate of a pain sensation, it can be successfully employed as a useful tool for investigating some aspects of the human nociceptive reactions in both experimental and pathological situations.