['The goal of this study was to investigate the role of nitric oxide synthase (NOS) in occlusal force-induced signal transduction in rat periodontal ligament (PDL).', 'Rats were fitted with a bite plate and a metal cap to the maxillary and mandibular incisors, respectively, to eliminate the occlusal forces on rat molars. One group was sacrificed at 7 days (exclusion group), while the remaining rats had their appliances removed to reestablish molar occlusal contact (reload group) and were sacrificed 7 days thereafter. Another group of rats (normal group) were left completely untreated. Frozen cross sections of the upper first molars were stained with NADPH-diaphorase to quantify NOS activity. The distal sides of the disto-palatal roots of the upper first molars were examined, and the number and the area of stained cells in the PDL were measured.', 'In the normal group, NOS expression was detected in blood vessels, monocyte-macrophages, fibroblastic cells and osteoclastic cells. NOS expression was lower in the exclusion group when compared with the normal group or the reload group (p < 0.05), and the exclusion group exhibited occluded blood vessels and a narrowing of PDL. In contrast, in the reload group the PDL and blood vessel structure had recovered and NOS expression was increased to the level of the controls.'] Is nitric oxide synthase expression increased by occlusal force in rat periodontal ligament?