Objective       To determine  the effects of millimeter wave (MMW) irradiation on substance P and histomorphology in rat skin.       Methods       A total of 126 health adult SD rats were randomly divided into MMW irradiation group (n=117) and normal control group (n=9). The outside skin on the back of MMW irradiation rats was radiated by 35 GHz MMW (40 W/cm

2

) for 30 s

and 1 and 3 min respectively. Rat skin temperature was detected before and after irradiation with infrared thermometer. The skin samples were taken at 0

5 and 10 min

and 1 h after irradiation for detecting substance P (SP) level with radioimmunoassay (n=6 for each time point)

and taken a

t 1

3

6 and 24 h after irradiation for observing histomorphology with HE staining (n=3 for each time point).       Results       MMW irradiation resulted in an obvious elevation of local skin temperature

  and collagen congestion and local degeneration necrosis at different degrees in the epidermal and dermal layers of the skin. The skin injury was deteriorated with elapse of irradiation time. SP level was increased significantly at 5 and 10 min after MMW irradiation

but it returned to the level of normal control at 1 h.       Conclusion       Based on our experiment condition

MMW irradiation induces the rat skin damage and hyperalgesia

which may be related with markedly elevated skin temperature (thermal effect) and release of SP.