Toph's in Chemical Mutagenesis is a new series dedicated to studies in the areas of environmental chemical mutagenesis and genetic toxicology. In this series we will explore some of many topics that are emerging in these rapidly developing fields. The purpose of the present volume is to attempt to organize and compare the genotoxic properties of the N-nitroso compounds. This is a particularly interesting class of compounds because of the problems encountered with the Salmonella assay of Ames in generating both false positive and false negative results. The battery approach using a number of assay systems seems more appropriate to evaluate chemicals in this class. Topics to be discussed in other volumes in this series include single-cell mutation monitoring systems, the detection of genetic damage in mammalian germ cells, the mutagenicity of pesticides, problems in monitoring human populations in genetic toxicology, and a glossary of terms in genetic toxicology. All of these books are in various stages of development and should appear within the next few years. Frederick J. de Serres Series Editor vii Preface During the past ten years there has been an explosive development in the number of short-term tests to predict the biological risks, especially risks of cancer, in exposure to xenobiotic chemicals. The number of published articles in this area has reached many thousands a year and there are several new journals devoted almost entirely to the presentation of the results obtained in these tests.