The periapical response to injury is a complex interaction of inflammatory, immune, neural, vascular and synthetic activity. Nitric oxide(NO), synthesized by nitric oxide synthetase(NOS) from L-arginine, is becoming recognized as an important bio-regulatory molecule in a variety of tissue, but little is known about its possible role in periapical tissue. The purpose of this study was to investigate the expression of nitric oxide synthetase(NOS) in tooth follicle, periapical abscess, granuloma and cyst. The expression of NOS in periapical lesions was evaluated by immunohistochemical staining for $NOS_2$, and $NOS_3$. The immunoreactivity was evaluated by staining intensity, and inflammatory cell infiltration. Correlationship between the periapical lesion in immunoreractivity were statistically analyzed by SPSS. The degree of $NOS_2$ and $NOS_3$ expression in periapical abscess was higher than in any other periapical lesions, and stastically significant. The expression degree of $NOS_2$ and $NOS_3$ was not correlated with periapical abscess and granuloma, but expression of $NOS_2$ showed very significant in periapical cyst. The increased expression of $NOS_2$ and $NOS_3$ was correlated with inflammatory cell infiltration degree of the periapical cyst. These results suggested that NO should play an important role in progress and/or mediation of periapical lesions.