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Twin studies are useful devices to determine the heritability of persistent but variable characteristics that tend to differ among individuals. Drug responses are not persistent affairs; they are temporary characteristics. One therefore may ask whether twin studies are necessary to assess the genetic element in pharmacological responsiveness. To measure the genetic component contributing to their variability, it seems logical to investigate the response variation by repeated drug administration to given individuals, and to compare the variability of the responses within and between individuals. We attempt here to describe a theoretical background of this venture, and to show some results of the exercise. Potential sources of error or uncertainty are discussed.