AIM:To observe the antineoplastic effect of Naja Naja atra venom (NNAV) on S180 bearing mice and to study the possible antineoplastic mechanism. METHODS:We observed the effect of NNAV on tumor weight、plasma nitric oxide content、plasma endothelin content and spleen index in S180 bearing mice with different concentration and different period by means of injecting into abdomen. RESULTS:Treatment with NNAV solution of different concentration could markedly inhibit S180 growth (especially in the low concentration group and by long period) and rate of inhibiting ranged from 21 63% to 49.25%; the plasma nitric oxide content, the plasma endothelin content and NO/ET ratio in tumor bearing mice were obviously higher than those of the normal control group, while after treatment with NNAV solution, the plasma nitric oxide level, the plasma endothelin level and NO/ET ratio could be reduced markedly, and it was noticed that NO/ET ratio in the group with highest inhibiting rate was most close to that of the normal control group. The spleen index was obviously increased after treatment with NNAV solution.CONCLUSION:The antineoplastic effect of NNAV on S180 bearing mice is best in long period by means of injecting into abdomen with low concentration. The mechanism of the antineoplastic effect of NNAV may be related to lowering the plasma nitric oxide and endothelin level, regulating the NO/ET ratio and enhancing the immune response.