Most GPs are part of a network of practices, which provide care during the evening, night or weekend, assuring that medical support can be given when needed.
If you call your doctor’s practice after regular opening hours the answering machine will tell you (often in Dutch) how to reach the doctor on duty in your area. Pharmacies also have this procedure during off hours.
In many cases there is a central office (Centrale Huisartsen Post) from where doctors (in turn) operate during off hours. These after hour services can only be used in case of an urgent medical situation that cannot wait until regular office hours.
During your first visit to the doctor’s office, you should be provided with details on what to do in case of a medical emergency during or after office hours. If not, make sure to ask!