Seventy-two canine mammary tumors were classified into 20 histologic subtypes which included 63 malignant tumors and 9 benign tumors. AgNOR counts were performed manually in all the tumor tissue sections. AgNORs which appeared as black dots were multiple, small, irregular shaped and arranged discretely or as overlapping dots in malignant tumors while in benign tumors, they were bigger, round and discretely arranged. The mean AgNOR number of mammary tumors ranged from 1.1 to 6.7 and was higher (4.58 ± 1.04) in malignant tumors than in benign tumors (1.4± 0.36). Among the different histologic subtypes, fibrosarcoma had the highest AgNOR number followed by anaplastic carcinoma, malignant myoepithelioma and the other histologic subtypes.