To express growth-related changes in physiologic or other functions in forms usable for kinetic modeling, we are interested in identifying regular relationships that take the form of simple mathematical expressions. Many anatomic and physiologic functions scale within or across species in accordance with the allometric relationship, y = axb. These include many organ weights, the glomerular filtration rate, respiration rate, oxygen consumption, or basal metabolic rate. The allometric lines may display discontinuities in slope associated with critical growth periods such as transitions from one growth phase to another. On the other hand, many other kinetically important processes depend on the physiocochemical characteristics of the agent and of the sites in the body with which it interacts. Their rates of development are determined by the age-dependence of these physiocochemical characteristics and of their interactions. Examples of different types of age-dependence are given, and their combined impact on the age-dependence of lead-kinetic behavior is examined.