Difference in concentration of the agent in regional tissue after ia. Administration (broken lines) as compared with iv. administration (continous lines), assuming that one part in four of the agent passing through the tumor circulation is biologically active against tumor cells (x=1/4). The upper scale indicates the difference in systemic blood concentration, which is minimal. Even through regional uptake is some 3 times greater after ia. Administration, in absolute values this is unlikely to be suficient to make a significant difference in systemic blood concentration. The abscissa indicates the time scale, measured by number of circulations. In the clinical situation the slope of the curves will be similar to those shown here but the the time scale will be more protracted, as some of the agent will move temporarily into the tissue fluid and subsequently return unchanged into the circulation, resulting in delayed detoxification or excretion. (by Stephens FO)