Environmental influences can significantly affect (both positively and negatively) the developing child. Three ways in which the environment affects the developing organism are induction, facilitation, and maintenance. In the case of induction the presence or absence of specific stimuli totally determines whether or not a particular developmental event occurs, such as sexual differentiation. Facilitation is apparent when the rate and/or maximal level of maturation is altered, as in the case of the influence of thyroid hormone on the rate of cell division. The term maintenance refers to the presence of any environmental factors necessary for the preservation of an already developed state. Alternately, and in particular reference to human development, certain aspects of the environment, such as toxic compounds, heavy metals, pesticides, food additives, and parental drug abuse may have predominately negative effects on the developing child. Although every part of the developing organism is subject to this environmental influence, in recent years considerable emphasis has been placed on the developing nervous system. As a result of scientific investigations, significant advances have been made toward understanding the consequences of toxic exposure. Future research should increasingly emphasize basic research approaches designed to reveal how toxicants produce their effects. In addition, special attention must be given to the developing organism in any assessment of the consequences of neurotoxic compounds. (Author/RH)