Abstract: A trial and error method for measuring dimensions of the vagina and specifically the effective diameter of the vaginal sphincter muscle of an animal such as a dog or other female animal, using a plurality of measuring devices having a nose cone portion with a blunt free end mounted to an elongate handle, said nose cone portion including an intermediate ring of maximum diameter. The sphincter muscle diameter is determined by inserting said nose cone portion through the vulva until said intermediate ring passes anterior to the sphincter muscle and thereafter repeating the step of insertion with different measuring devices having increasingly larger diameters until a marked build-up of resistance to anterior movement of the device is detected upon contact of said intermediate ring with the sphincter muscle.