Information that can be used to assess trends in the health of the population is limited to the results of irregular surveys of nutritional status and 'I.Q.', to data obtained from the notification of infectious diseases, congenital malformations, blindness and other selected defects, and to mortality rates. The last have been recorded since 1841 and provide the most detailed and useful information, although they are often difficult to interpret because of changes in the nomenclature, classification, methods of diagnosis, and efficacy of treatment of disease states. In the last 40 years, mortality rates have shown progressive reductions at all ages which have continued past the time when improvements in the prevention and treatment of infectious disease might be expected to have produced their principal benefits. Notable differences have emerged between the sexes, the rates continuing to decline in women but remaining more or less stable for a period in middle-aged men. This difference can be attributed to sex differences in life-style, so that until recently the trends in women are likely to have been the better indicators of the effect of toxic agents in the environment. The available data are inadequate to assess possible effects such as alterations in behaviour, but are of some help in regard to teratogenicity and carcinogenicity.