【Abstract】ObjectiveTo observe the effect of nimesulide (NIM) on DMBA-induced mammary tumors and to investigate possible mechanisms of inhibiting tumors. MethodsThe Wistar rats were randomly divided into four groups: DMBA group, NIM+DMBA group, NIM+diet group and diet group. The incidence and mean latent phase of mammary tumors were observed. The number and volume of tumors in every rat were measured. The apoptosis index and proliferation index were evaluated by TUNEL assay and PCNA immunohistochemical staining respectively.Results The latent phases of mammary tumors in NIM+DMBA group were strikingly longer than those in DBMA group 〔(115±14.8) d vs (84±15.6) d,P＜0.01〕. The incidence of mammary tumors was 69.2% in DMBA group, 46.4% in NIM+DMBA group. There was obviously decreased incidence in NIM+DMBA group (P＜0.05）. The number and the mean volume of mammary tumors in every rat in NIM+DMBA group were markedly less than DBMA group 〔（0.82±0.31） vs （1.52±0.43），（3.24±1.21） cm3 vs （6.03±1.84） cm3，P＜0.05〕. The apoptosis index was obviously higher and the proliferation index was markedly lower in NIM+DMBA group compared with DMBA group〔(42.54±13.24)% vs (18.14±6.66)%, (20.25±4.97)% vs (36.36±5.02)%, P＜0.05〕. ConclusionNIM can inhibit the development of DMBA-induced mammary tumors by inhibiting proliferation and inducing apoptosis.