Grade X
Procedures which cause severe, inescapable or unrelieved anxiety, fear, pain or distress where the intensity or duration, or both, of the induced suffering are at or beyond the limits of reasonable endurance.

For instance: conducting major surgeries without the use of anaesthesia (e.g. where the animal is immobilised physically or with muscle relaxants); testing the effectiveness of pain-killers (analgesics) in animals with induced severe pain.

Very severe suffering or noxiousness. Procedures require the most exceptional justification.