Primary vacuum systems of the Electrotechnical Laboratory (ETL) and the Istituto di Metrologia ''G. Colonnetti'' (IMGC) were compared in the pressure range 10^-4 Pa to 10 Pa by using spinning rotors as transfer gauges. Several rotors were calibrated at both laboratories in an argon atmosphere. The results obtained are in good agreement and are within the uncertainty values of both the laboratories.