The author's previous theoretical study described the determination of a physiologically realistic structure of a mean residence time of an intravenously administered drug (Ďurišová 2012). This study continues previous work and the aim is to determine physiologically realistic structures of mean residence times of a drug administered either orally (MRTpo) or intramuscularly (MRTim). The determinations are based on the following assumption: a cardiopulmonary, portal, portal-venous, hepatic-portal subsystem, and a subsystem that mathematically represents non-eliminating tissues can be considered to be most important in terms of their impact on MRTpo and MRTim. If drug fate and disposition is a linear process, the used method allows developing mathematical models without any prior knowledge or hypothesis concerning drug fate and disposition. This is a great advantage of the method considered here when compared with compartment methods. The research presented in this study was aimed at contributing to the knowledge base of physiological origins of mean residence times.