Hyperthermia inhibits cell division and progression of the cell cycle in normal tissue and causes inactivation of the enzymes which can effect the rejoining of DNA breaks. This leads to metabolic derangement and cell death in cancer cells. Hyperthermia using microwaves may destroy the cancer cells, but cause slight reversible changes in normal tissue. To evaluate the possibility of interstitial Nd : YAG laser hyperthermia as an adjuvant treatment modality in head and neck cancers, the author observed morphologic changes by light microscope and transmission electron microscope and by measuring the tissue level of inflammatory chemical mediator, prostaglandin E₂(PGE₂), in the effects on normal rabbits' tongue mucosa after interstitial Nd : YAG laser hyperthermia. The results were as follows : 1) On light microscopical observation, basal keratinocytes and basement membrane were destroyed partially on the first day, they were regenerated partially on the third day, and they were regenerated completely on the seventh day after interstitial Nd : YAG laser hyperthermia. 2) On transmission electron microscopical observation, basal keratinocytes showed karyorrhexis and aggregation of cytoskeletal elements and basement membrane was destroyed on the first day, basal keratinocytes showed cytoplasmic blebs and swelling of mitochondrias and basement membranes regenerated partially on the third day, and the basal keratinocytes and basement membrane regenerated by the seventh day after interstitial Nd : YAG laser hyperthermia. 3) The PGE₂ levels were increased above normal control on the first day, highest on third day and normalized on the seventh day after Nd : YAG laser hyperthermia. Therefore, interstitial Nd : YAG laser hyperthermia caused minimal and reversible changes on a normal rabbit's tongue mucosa and inflammatory reaction showed only seven day after hyperthermia. On conclusion, interstitial Nd : YAG laser hyperthermia caused a little and reversible changes on the normal tongue tissue. Therefore, author expects to provide basic data for using interstitial Nd : YAG laser hyperthermia as an adjuvant treatment modality preserving organ function in the head and neck cancers.