Members of the trade union Unite at Yorkshire Ambulance Service are taking industrial action today and also on Monday over the introduction of longer shifts.
The strikes will be held from 3pm to 8pm. Union leaders say the new hours could mean staff working for 10 hours without a meal break. They want a protected break of 30 minutes after six hours.
Members previously walked out four times in February, and twice last year.
Yorkshire Ambulance Service NHS Trust said it continued to seek on-going dialogue with Unite through Acas.