The effects of a repetitive stress induced by anticipation of pain were studied on the following somato-vegetative parameters: monosynaptic reflex (H reflex), nociceptive flexion reflex (RIII reflex), heart rate and respiratory frequency, in normal trained volunteers. A progressive increase in heart rate and in respiratory frequency, as well as a facilitation in H reflex parallel to an inhibition of RIII reflex (threshold increased) were observed in all subjects as a function of repetition of stress during 45 min. At this moment, a double-blind injection of a strong dose of naloxone (4 mg) resulted in a rapid exacerbation in vegetative responses (tachycardia, polypnea) as well as in an increase in the facilitation of the H reflex, whereas the RIII reflex was dramatically facilitated (threshold decreased). In contrast, no significant change in these parameters was observed during placebo (saline) injection compared to a control situation (no injection). These data strongly suggest that endogenous opioids are involved in the phenomenon of stress-induced analgesia in man. Moreover, they show that the release of opiates in these conditions possibly modulates non-specific structures which are known to be activated during stress or emotion.