Abstract: Mammary tumours rank second as the most common neoplasms in dogs after skin tumours, whereas in women the most common cause of cancer-related deaths is breast cancer. N-Methyl-N-Nitrosourea (NMU) is a highly specific mammary gland carcinogen which directly act and does not require metabolic activation. In the present study, NMU at the dose rate of 50 mg kg-1 body weight was used intra-peritoneally for the induction of mammary tumour. The first palpable tumour appeared on 70th day post carcinogen injection and subsequently, most of the tumours were developed around 18-20th week. During 28 weeks of experimental period, the tumour incidence was 82.86% (29/35). The tumour frequency was found to be 4.7±0.33 tumours and the average latency period was 107±4.1 days. The average tumour volume was found to be 69±8.8 cm3. Equally, 50% of mammary tumours appeared on the right (22/44) and left (22/44) mammary gland chain. Region wise, 81.82% (36/44) of the tumours appeared on abdominal-inguinal mammary glands and 18.18% (8/44) on the cervical thoracic mammary glands. A total of 44 mammary tumours were diagnosed in which 88.64% (39/44) were malignant and 11.36% (5/44) were benign. Among the malignant tumours, 33.33% (13/39) were non-invasive and 66.67% (26/39) were invasive. The average values of mitotic index, Proliferating Cell Nuclear Antigen (PCNA), Vascular Endothelial Growth Factor (VEGF) and Platelet Endothelial Cell Adhesion Molecule (PECAM-1) in NMU induced mammary tumours were found to be 4.5±0.46/hpf, 77±2.6, 16.2±0.86 and 15±0.69, respectively. The present study for the first time demonstrated the expression of VEGF and PECAM-1/CD-31 proteins in NMU induced mammary tumours.