Abstract: The chamber comprises a tubular body terminating in front in a part shaped as a truncated rectangular cone. Two elastomeric rings are for adjusting around the snout of an animal, such as a rat or guinea pig. A piston bounds the space occupied by the animal's body. A collar disposed between the two rings comprises two annular segments each secured to a rod extending radially towards the exterior and secured to an actuating means and associated with a cam. A spring constantly urges the segments towards the interior. The collar is for adjusting behind the animal's head to prevent it from moving back.