an apparatus for producing hollow bodies , in particular bottles and similar containers provided with an evacuation opening , of thermoplastic includes a lower machine part and a beamlike upper machine part . the lower machine part is equipped with at least two blow molds , each provided with a cavity . a number of blowing and calibration mandrels corresponding to the number of blow molds is disposed on the beamlike upper machine part disposed above the lower machine part . one blow mold is assigned to each blowing and calibration mandrel . the blowing and calibration mandrels can be axially fed jointly toward the blow molds and can be moved with their calibration region into an orifice of the cavity until an annular shoulder of the blowing and calibration mandrels comes into contact with a counterpart face on the associated blow mold . the blowing and calibration mandrels are retained axially freely movably , within predeterminable limits , in the upper machine part . upon joint feeding of the blowing and calibration mandrels toward the blow molds , the blowing and calibration mandrels are automatically adjustable in height relative to their outset position , counter to the resistance of a coupling medium .