I tried this exact item a few years ago. I was very hopeful that it would work well. I wanted to be able to take my guinea pigs out on the green grass in my backyard so they could get some fresh air (only in nice weather of course).I have never owned a ferret, so I can only speak for the use on guinea pigs.My guinea pigs and I went through much distress trying to make this "leash" contraption work. It didn't. EVEN when I DID get it on, it wouldn't stay on.There's no reason to subject any guinea pig to that type of stress. They hate things wrapped around their body in a "tight" fashion, and this particular leash has to be somewhat tight. There's a possibility it could damage their delicate lungs, so please reconsider using this on your guinea pig.Maybe it works better on ferrets, but it's a "NO GO" for my two guinea pigs.