Nitric oxide (NO) is an important regulatory molecule in bone formation and resorption. The purpose of this study was to examine the role of NO in orthodontic tooth movement in rats. We used specific inhibitors of NO synthases (NOS). Upper first molars of 9-week-old male Wistar rats were moved buccally for 21 days. The local administration of N(G)-nitro-L-arginine methyl ester. HCl (L-NAME), a general inhibitor of NOS activity, significantly reduced tooth movement. On the other hand, N(6)-(1-iminoethyl)-L-lysine. 2HCl (L-NIL), a selective inhibitor of the inducible isoform of NOS, had no effect. These results suggest that NO is an important biochemical mediator in the response of periodontal tissue to orthodontic force and is produced primarily through the activity of constitutive NOS.