Substances are usually tested on conscious animal subjects.  Animals are either forced to ingest poisonous substances or are lathered in dangerous chemicals. Painful and Deadly Tests states the following:

In the Draize test, caustic substances are placed in the eyes of conscious rabbits to evaluate damage to sensitive eye tissues. This is extremely painful for the rabbits, who often scream when the substances are applied and sometimes break their necks or backs trying to escape the restraints. In the LD50 test, subjects are forced to ingest poisonous substances (through stomach tubes, vapor spray inhalers or injection) until half of them die. Common reactions to LD tests include convulsions, vomiting, paralysis and bleeding from the eyes, nose, mouth or rectum.