The invention relates to a method for producing blow-moulded hollow bodies. Flat preforms (1, 1') are produced during the working cycle of a blow-moulding installation, said preforms emerging from wide-slit nozzles (2), and being connected, while still in the thermoplasticised state in the blow-moulding installation, along a pinch-off weld formed by the closing of the blow-mould (3), and expanded to form a hollow body (4). So that the flat preforms assume a reproducible position both in relation to the respective blow-moulding half (10) and to each other, a plurality of measures according to the invention are carried out alone or as a group. The synchronism of the preforms emerging from the wide-slit nozzles is monitored. If there are variations in the synchronism behaviour, the flow resistance is modified in order to correct the synchronism by heating the flow channels differently and/or by actuating a throttle element (17) acting on one of the partial flows, or the transport power of a plasticising unit (5) is adapted.