Female Wistar rats of 50 days old were administered with five doses of 50 mg/kg body weight N-methyl-N-nitroso Urea (MNU) intraperitoneally at weekly intervals. Animals were physically examined for tumour induction twice weekly throughout a 25 week observation period. Both malignant and benign mammary tumours developed in five out of eight rats. Most of the mammary tumours were observed in the abdominal-inguinal mammary gland than cervical-thoracic mammary gland. The presence of the tumour was confirmed by physical examination and histopathology. We conducted zymography for detecting the activity of matrix metalloproteinases (MMPs) in both malignant and benign mammary tumour and also normal organs like heart, kidney, liver, spleen and blood from tumour bearing rats. The results indicated that, the activities of MMPs were similar in malignant and benign tumours. MMPs activities were higher in tumour tissues than non tumour tissues like blood, liver and spleen of MNU-treated rats. In heart and kidney no MMPs activities were observed. MNU injection resulted in mammary tumours with good reproducibility as witnessed by gross and histopathological observation. Further, the MMP activities were higher in tumour tissues which may help in tumour development and progression.