Although this Conference focuses on the fetus and child, it should not deflect us from the larger issue-effects over the total lifespan. Early developmental stages are important less in themselves than in what they portend. So that you do not conclude, to our immense embarrassment, that we are about to envelop you in a Freudian mist, let us immediately brace these statements with some quantitative perspectives. Assume that exposure to an environmental contaminant takes place over a lifetime-a contaminant that, like methylmercury, can destroy brain tissue (although we don't know whether a threshold for damage exists for methylmercury). What are the implications for central nervous system function and performance? Figure 1 represents extrapolations from a function postulated by Kety,1 on the basis of the then existing literature, to represent aging of the brain. It shows a decline in neuronal cell density (paralleled by declines in oxygen consumption and blood circulation) of about 20% to 25% between age 25 and 75. There is substantial evidence that the central nervous system undergoes a continuous loss of cells throughout postnatal life, that a large proportion of cells die during morphogenesis, and that numerous structural and chemical changes take place both in the developing and the aging brain. An important, in fact a key, concurrent process is the decline with advancing age in a wide range of behavioral performance measures. Weiss and Simon2 calculated the additional decrease that would be imposed by enhancing the rate of loss calculated by Kety. Even as minute an increment as 0.1% represents a significant acceleration in terms of equivalent brain age.