Ambulance staff in Merseyside and Cheshire are taking industrial action in the latest of a series of strikes.
Ambulance technicians fear a pay cut as well as lost bonuses
The Ambulance Service Union technicians and high-dependency staff are staging a 12-hour walk-out on Saturday.
The industrial action, which starts at 1900 BST, follows a row over changes to NHS pay bands.
Paramedics are not taking part in the dispute and contingency plans are being implemented by North West Ambulance Service NHS Trust during the walk-out.
Technicians - who assist paramedics - fear the restructuring of NHS wage bands will lead to a pay cut, as well as lost bonuses.
The new system introduces a new basic salary with additional payments for working unsociable hours, but the Ambulance Service Union believes the change equates to a cut in the hourly rate paid to technicians.
However, North West Ambulance Service NHS Trust said it has pledged £1m to develop the skills of about 90 technicians to allow improvements in salary over a period of time.