The Gas Adaptors for Stereotaxic instruments, manufactured by Stoelting, enable gas anesthesia of rats or mice in a Lab Standard? or other stereotaxic instrument, without exposing the researcher to the gas. The Gas Anesthesia mask is installed by removing the nose clamp from the standard rat adaptor, and sliding the adaptor over the incisor bar, with the cone opening toward the open "U". The animal (rat or mouse) is installed by placing its teeth over the incisor bar, and sliding the adaptor forward until the cone is snug about the animal's nose, thereby substituting for the nose clamp. The teeth and the cone provide a snug, stable hold.