Abstract: Biaxially stretched thermoplastic tubes are made in an apparatus having a plastic extruder, the output of which is controllable on an open-circuit or closed-circuit basis and which has a too head for the extrusion of a highly viscous exiting tubing, a stretching mandrel and a speed-controllable draw-off device. In the region of the run-on part there is disposed a surrounding temperature control bath for controlling the temperature of the tube to the stretching temperature while a cooling bath is disposed in the region of the draw-off part. A Raman spectrometer operating with laser waves is disposed in the zone downstream of the cooling bath. The frequency of the laser waves is adapted to the plastic used and the energy of the laser waves is so adapted to the wall thickness of the tube produced that in addition to the radiated wave scattered or reflected by the plastics, at least one wave corresponding to a Raman spectral line is radiated from the tube.