Although animals are relatively expensive experimental models compared to the alternative methods discussed later, there are several important reasons for their continued use. First, there is extensive information available on their normal biochemical and physiological properties. Seconds, the published-data from the measurement of toxicological endpoints in vivo, using models of relevance to humans and domestic/farm animals, makes animal testing a valuable tool to predict toxicity. The animals that are most commonly used in toxicological testing are rodents and rabbits (Table 5.1).