The gas anesthesia mask is installed 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. Anesthetic gas flows in the tubing connector, past the animal’s nose, and out the other side. Spent gas may be routed to recycling, exhaust or gas analysis instruments.