Hundreds of Loudoun residents will line up at a school in Potomac Falls on Saturday to be treated amid a deadly anthrax attack.

But there is no need to worry: The attack is imaginary, and the residents will be volunteers, not victims, in Loudoun County's first emergency preparedness exercise.

The drill, organized by local agencies, is designed to test the county's system for quickly dispensing medicine to a large number of people, said David Goodfriend, director of the county health department.

"If, God forbid, there's ever a disaster, either natural or man-made, we need to be prepared," said Francis Rath, a medic who plans to participate in the drill.

The exercise, "Potomex '05," has been in the works since November. Officials throughout Northern Virginia have begun planning similar drills in recent weeks to test their emergency preparedness, and the state is getting ready, too: Gov. Mark R. Warner (D) has ordered the first state plan on dealing with an emergency evacuation.

The efforts come at a time of increased concern over the spread of avian flu, reported this month in birds in Romania and Turkey, and in the aftermath of Hurricane Katrina, which underscored the importance of emergency preparedness, Goodfriend said.

"One of the main things we learned . . . is that it's great to have a plan, but if you don't test and improve upon it, it's not going to be worth that much," he said.

The choice of anthrax as the cause of the mock emergency was no accident. Loudoun has a history with that particular threat.

Four years ago, in the weeks after the Sept. 11, 2001, terrorist attacks, worries about anthrax as a biological weapon seemed confirmed when people in Florida, New York and Nevada were exposed to the bacteria. Spores were found in the U.S. Senate, forcing the shut-down of the Capitol.

Then a contract worker in Sterling who handled State Department mail was hospitalized with symptoms of anthrax infection just days after two postal workers in the District died from anthrax disease. David R. Hose Sr. of Winchester survived, but the county health department immediately treated the other employees with antibiotics.

"We chose anthrax because it is something that happened in our county, and it is real for residents who lived through it," Goodfriend said.

Organizers say they hope they'll learn from this year's scenario by making it as realistic as possible. That means hundreds of "patients," hundreds of pills -- in this case, jelly beans -- and a few surprises.

"In a real emergency, we know there are going to be some bumps in the road, so we're purposely injecting some problems this time," Goodfriend said.

Those problems may include an unruly patient or one with significant health problems.

Patients will go through a triage station, where medics will separate patients according to levels of illness, and a station where they will fill out personal information forms to alert medics to allergies and prescription medicines. Medics will also dispense "pills" and answer questions.

After the drill, the volunteer patients will receive complimentary battery-powered radios -- to be used in case of a real emergency, Goodfriend said.

Organizers still need volunteers to reach their goal of 1,000 patients. The more volunteers, the more realistic the simulation can be, Goodfriend said.

"If only 100 people show up, it's not going to be a good test," he said. "For people who go through the stations, it's going to be that much easier to know what to expect in case of a real event."

Potomex '05 begins at 10 a.m. Saturday at Potomac Falls High School, 46400 Algonkian Pkwy., and participants should expect to be there two to three hours. To volunteer, register at www.loudoun.gov/health/volunteer.htm.