Tanzania Veterinary Association Proceedings Vol. 38 (2020)Canine transmissible venereal tumour (CTVT) is a neoplasia naturally transmitted in susceptible dogs through coitus. CTVT has a worldwide distribution, with a high prevalence in tropical and sub-tropical urban environments. The study aimed at evaluating CTVT lesions in local breeds of dogs and to assess morphological changes based on sex before and after administration of vincristine sulphate. Clinical and gross morphology, fine needle aspirates cytology (FNAC) and routine histopathology methods were used. Two FNAC and histopathological microscopic slide sections from each of the seven sampled dogs were stained with Giemsa stain and Hematoxylin and Eosin. All dogs were treated with vincristine once weekly over a six weeks period after which clinical morphological and histopathological changes were assessed. Grossly, before treatment the tumour masses appeared irregular, cauliflower like with tendency to bleed, sizes ranged from ≥5cm to ≤2cm with or without metastasis to regional lymph nodes. Cytologically, the tumours had homogenous, sheet-like cellular mass. Cytoplasm with punctate vacuoles, anisokaryosis with anisonucleoliosis and coarse to reticulate nuclear chromatin were seen. Lymphocytoid cell pattern was dominant cell type. Histopathology showed sheets of round cells with nuclear and cytoplasmic variations. Histopathology of the treated dog revealed hyper- cellularity, absence of nucleoli, prominent mitotic figures, reduced cell size and presence of inflammatory cells. There was no difference on the cellular changes after vincristine treatment between female and male dogs. Cytology and histopathology showed that vincristine sulphate suppresses the development of tumour through alteration of cellular morphology with no difference between male and female dogs.