In addition to the variety of extrinsic (e.g., environmental, nutritional and experimental) factors which can influence the kinetic and dynamic behavior of non-essential metals, certain intrinsic or constitutive characteristics of the host organism can influence the distribution, metabolism and toxic effects of these metals. An important task in metal toxicology is identification of these constitutive factors and elucidation of the molecular bases of organismic differences in the kinetic and dynamic behavior of non-essential metals. Understanding of these differences increases the knowledge of the physiological processes controlling metal disposition and improves the accuracy of the assessment of hazard associated with exposure to non-essential metals. The incorporation of these constitutive factors into the modelling of the kinetic behavior of non-essential metals will improve the descriptive and predictive value of the models.