Guineapigs HATE those leads ! A friend of mine, a few years ago, 'kindly' bought me one of those guineapig harnesses and I felt obliged to give it a try. (knowing that is was a pretty daft idea, but she was stood there). The guineapig went mental when it was in the harness and leash, it desperatley tried to leap about to get it off and dashed in random directions..when it got to the end of the length of the lead it jerked wildly to a halt. Guineapigs walk where they want to walk, they aren't suited to being 'taken' places like a dog, they don't need the exerscise anyway as they 'walk' themselves. The most you might achieve is having the guineapig 'walk you', it'll go where ever it wants and you will follow on the end of the leash, the worst you will end up doing is - god forbid - dragging it along and hurting it.