I have a six month old female Shih tzu/yorkie mix that pants excessively. She has done this as long as I have had her (since she was six weeks old). She is not overweight and the panting is not from being excited or out of breath. She can simply be lying down with me and be breathing very rapidly with her mouth open. I have never asked her vet about it; it seems to slip my mind when she is getting shots...anyway if anyone knows any reason she might be doing this please let me know. I am worried she may have some kind of heart defect or something. Maybe i'm just overly concerned - i don't know.:dogtongue2: