During unclassified procedures the animal is placed under general anaesthetic before the start of the procedure, and is humanely killed without ever regaining consciousness.
A sepsis (toxic shock) study in a mouse under 'terminal anaesthesia'
"The mouse is given a general anaesthetic. A surgical procedure is carried out which involves making a hole in the intestine (gut) so that the contents leak into the body cavity and have a toxic effect. A potential treatment for sepsis (toxic shock) is tested while the mouse is deeply under anaesthesia and very carefully monitored. Once the experiment is over, the anaesthesia is increased until the mouse dies."
Example taken from Openness on Animal Research Dialogue.