Midbrain periaqueductal gray mater area (PAG) receives some afferents from anterolateral system, frontal cortex, and hypothalamus that inhibit pain through its descending pathway that synapses medial raphe nuclus (mrph) and finally terminates on spinal cord enkephalinergic interneurons in layers II and III. In the present study, the role of Ca+2–channels located on medial raphe nucleus neurons was studied. For this purpose, thirty male NMRI rats weighting 185-240 g were used and a stainless steel guide cannula (0.7 mm in diameter) was implanted into mrph nucleus under stereotaxic surgery (AP = -7.8, ML = 0, DV = 8). Then, one microliter of deltiazem(0.1 mg/kg) was injected into mrph. Five minutes after injection, the pain was evaluated using formalin test (50 µl of 2.5% formalin solution was injected into hind paw, s.c.). Every 15 seconds of each 5-minute blocks was scored by 0-3 scores. Sham- treated animals received the same volume of ACSF into mrph nucleus. These results show that intra mrph nucleus administration of diltiazem could not significantly change pain threshold in male rats.