The aim of this study was to determine the changes of the nociceptive thresholds in response to an acute mechanical stimulus (paw pressure) in magnesium (Mg)-deficient rats, and the involvement of the NMDA receptor in these changes. Changes in vocalization thresholds was determined after 7 days of feeding with a Mg-depleted diet. Compared with the control group, Mg-deficient rats showed a significant decrease in the vocalization thresholds (-35.8 +/- 2.5%, p < 0.001) reflecting hyperalgesia. In Mg-deprived rats, three doses (0.06, 0.12 and 0.24 mg/kg s.c.) of dizocilpine (MK801), a non-competitive NMDA receptor antagonist, significantly reversed the hyperalgesia in a dose-dependent manner for at least 48 h. No effect of MK801 was observed in the control group. These data provide evidence that Mg deficiency could constitute a new model of hyperalgesia involving NMDA receptors.