Interactions of various agents are important considerations in developmental toxicology. Several recent papers have reviewed such interactions, with a focus on multiple chemical exposures. The purpose of this paper is to discuss the potential confounding effects of multiple exposures on developmental toxicity--both in epidemiological evaluations and experimental studies. Not only must concurrent exposures to multiple chemicals be evaluated, but exposure influences may be separated in time. Personal habits and factors such as age, genetic makeup, diurnal variations, nutritional imbalances, and metabolic disturbances may alter an individual's response to a particular agent. In addition, both micro- and macroenvironmental conditions such as altered ambient temperature, noise level, "stress," and proximity to atmospheric or dietary pollutant sources, may impinge on the adverse effects of target developmental toxicants. Leisure time as well as occupational exposures must be evaluated. Teratologists must be alert to the many extraneous variables which may interact to affect the developmental toxicity of individual agents.