The article discusses scientific researches that reveal mechanisms of intraspecific differences in responses of organisms to chemical exposure. Factors of individual sensitivity to toxicants are represented by genetic differences between individuals of the same species and genetically unconditioned factors. Genetic factors are mutations, including under-researched mitochondrial DNA mutations and genomic drift, genetic polymorphism, and gender-related differences. Age, conditions of keeping and state of organisms, seasonal variations of body functions, and biotic interactions are considered as factors that are not related to the genetic apparatus. The article considers a number of works, in which effects of the combined action of external factors on sensitivity of organisms in model groups are studied.