Abstract: The invention relates to a catheter assembly comprising a cannula, needle and catheter inserter wherein the inserter is in slidable relationship to the cannula and wherein the inserter or other device also serves to selectively restrict the cannula and needle from sliding relative to the inserter. The cannula comprises a material which, when inserted into the body of a living subject, softens and/or swells and has a tensile energy to break of at least about 700 N-cm/cm.sup.3 and a 2.5% Secant modulus of less than about 7,000 N/cm.sup.2. The invention also relates to a method of inserting a cannula into a living subject using an inserter which is positioned such that the desired length of cannula for initial insertion extends beyond the inserter. The method also relates to further inserting the cannula into the living subject after initial insertion of the cannula.