Introduction

Over the past 30 years, there has been a general change of emphasis in toxicology from the traditional concerns with poisons and antidotes to understanding the chronic toxic effects associated with exposure to environmental agents. The major cause of this shift was a focusing of public interest on long-term health effects of man-made chemicals. This public awareness was motivated in part by the successes of public health sanitarians in controlling infectious diseases (with the recent exception of AIDS), which resulted in the lengthening of life expectancy in the population (Hart and Turturro, 1990). This change in emphasis has radically altered the scientific methods Used in toxicology. There are basic differences in evaluating short-term and long-term consequences of toxic exposure. Interestingly, techniques similar to those used to understand the effects of long-term toxicity are also useful in understanding aging (Hart and Turturro, 1988). Factors that contribute to these differences include the comp