The purpose of present study was to evaluate the relationship between the early change of gingival condition and methyl mercaptan concentration during experimental gingivitis. Ten men(23-25 years old) whose gingiva were clinically healthy were selected. The participants have ceased to perform all forms of oral hygiene during 14 days and then did thorough plaque control for 7 days. For each subject, the methyl mercaptan concentration was measured by $B.B.Checker^{(R)}$ (Bad Breath Checker with printer, Tokuyama Soda Co.,LTD., Japan)before experiment and 1,4,7,14,21 days during experiment. Plaque index(Silness & $Lddot{o}e$), gingival sulcus depth and sulcus bleeding index($Mddot{u}hlemann$ & Son)score were recorded. The results were as follows. 1. Methyl mercaptan concentration increased continuously from the first day to the 14th day, decreased on the 21th day but it was still higher(P<0.001). 2. Plaque index score and sulcus bleeding index score tended to increase on the 4th day, markedly increased on the 14th day and returned to baseline level on the 21th day. 3. There was parallel relationhsip among methyl mercaptan concentration, plaque index score and sulcus bleeding index score. This result suggests that methyl mercaptan concentration increased with deterioration in gingival health, but decreased during recovery of normal health condition.