The drill takes on critical urgency in the wake of exposure to an irritant that sent five postal workers to the hospital last month. The San Francisco Fire Department is assisting in Wednesday's activities.
Officials are testing a bio-hazard detecting machine and coordinating evacuation procedures with the fire and health departments.
"We're going to be doing an evacuation that will be later this afternoon. This time, this morning this is postal inspectors working with the fire department, the health department; going in and pulling evidence from the system," said Postal Inspector Jeff Fitch.
The drill was previously scheduled and is not directly related to the May 9 incident where postal workers came into contact with a substance that had them itching and left them light-headed. Five people were sent to the hospital.
The substance was material from a frayed conveyor belt and was deemed not harmful.