The lab at Grey Bruce Health Services Southampton site will be closed Wednesday and Thursday.
Electrical services are being upgraded as part of the Emergency Room and laboratory renovation.
The closure will help minimize traffic at the hospital until that work is done. The lab will re-open on Friday.
The ER will remain open 24/7 as usual.
Phase one of the expansion project started in May and is expected to take two years to complete.
It includes a 10,000 square foot addition to the ER and major lab renovations.
The new ER and Ambulatory Care area will include 11 modern treatment rooms, and the lab will be doubled in size.