Abstract: A catheter or like instrument having a variable flexural modulus for insertion into the body includes a length of flexible tubing having a normally relatively low flexural modulus. A first lumen inside the tubing provides a free flow path therethrough. Associated with the catheter tubing for controllably increasing its flexural modulus is a second lumen having a closed distal end and a proximal end adapted to be connected to a pressure source. An increase in pressure inside the second lumen serves to increase the flexural modulus of the catheter tubing to stiffen the same particularly during insertion of the tubing into the body. After insertion of the same, the pressure is controllable so that the catheter tubing is allowed to return to its normally low flexural modulus.