The Department of Transportation is no place for glamour.

And so, for a visit there Friday, Michelle Obama pulled her hair back and got down to business.

There as part of a series of “pep talk” speeches at government agencies, the First Lady put on a signature brooch, a houndstooth blazer (by Chicago designer Peter Soronen) that she’s worn before and a hairstyle she doesn’t often choose.

The First Lady’s hair is usually styled straight or slightly flipped at the ends. But on a few occasions — once during last year’s Democratic National Convention, another time on the campaign trail on a boiling-hot July day in 2007, onstage with Oprah last February and at her wedding, in 1992, for example — she’s sported a ponytail or updo.

The White House, which fielded a slew of calls over the look yesterday, declined to comment.

When it comes to the most captivating First Lady of this generation, the slightest change in appearance makes waves — and garners fanfare from her loyal style followers.

“This up-do feels very soft, casual and easy,” says Mary Tomer, founder of Mrs-O.org, a blog dedicated to the First Lady’s outfits.

“It’s elegant without trying too hard. She’s got this short jacket on and this really bold broach – and so I think pulling her hair up keeps the whole look feeling very cohesive and polished.”

Washington insiders, perhaps unaccustomed to the power of a First Lady’s every style choice, expressed shock at Friday’s flurry of attention.

Exclaimed one D.C. source: “Jeez. She only pulled it back for one second.”