An evaluation of absorption after extravascular application of drugs, together with the evaluation of the global process of drug distribution could be considered as basic problems of pharmacokinetics. From the systems point of view, the mentioned problems have dual character and could be solved using the same approach. This paper is considered as presentation of an original approach to the evaluation of the absorption profiles. Being based on the theory of adaptive models it has some advantages in comparison with other approaches, mainly due to simple and robust algorithms without need of any special prerequisites.