"Our concern is preserving America's past," begins Lee H. Nelson. ". . . So people in the future can see the beauty of America's architecture," adds H. Ward Jandl.

The two men have worked together at the Interior Department since 1976, and when one starts a sentence, the other often finishes it.

They head a small National Park Service office that determines whether work on commercial buildings is historic rehabilitation -- and thus eligible for federal tax credits. If a building is listed in the National Register of Historic Places or located in a National Historic District, developers may qualify for a tax credit of as much as 25 percent of the rehabilitation costs.

Nelson, 57, is director of the preservation assistance division; Jandl, 38, heads its technical preservation services branch. They are responsible for certifying that the rehabilitation work is done in compliance with exacting federal standards and that the historic components of a building are not lost.

"Without the tax credits," Nelson said, "we'd have a lot more new buildings and . . . and . . ."

". . . And many great pieces of architecture just wouldn't have been saved," Jandl continued, "because of the profit motive in the private sector."

Nelson nodded in agreement, saying, "We have a preservation program, not a tax program. Our job is to certify that the work is done within the standards of historic rehabilitation."

Nelson learned the preservation trade at the University of Oregon, where he earned a bachelor's degree in architecture. After receiving a master's degree from the University of Illinois, he joined the government in 1958 as an architect with the Park Service's Baltimore office. There he headed a project that involved studying how Fort McHenry -- where the flag that inspired "The Star-Spangled Banner" flew -- had been altered over the years.

After another year in Yorktown, Va., Nelson was named head of a team that would spend the next 12 years repairing Independence Hall and the Old City Hall in Philadelphia. He said it was the most extensive and intricate structural rehabilitation of a historic building in the nation -- and "the high point of my life."

Afterwards, he asked to come here to teach his techniques to others. "I didn't want to come, but I shouldn't say that," Nelson said. "I always had a strong interest in hoping that the experiences of the Park Service could be shared with the rest of the world."

Shortly after that, in 1972, he was named head of a Park Service division that was designed to help other agencies preserve the properties under their control.

Jandl followed a different career path to the government. After graduating from Yale with a degree in architectural history in 1968, he joined the Peace Corps.

After graduate work in historic preservation, he went to work for the New York Public Library. In 1974, he joined the Park Service as a reviewer of projects proposed for the National Register of Historic Places.

The two men got to know each other then, and when the Tax Reform Act of 1976 established a credit for historic-preservation work, Nelson invited Jandl to head the office that would monitor the projects.

That law provided for a tax credit of 10 percent for rehabilitation work, and developers generally weren't interested -- it was still easier and more profitable to raze a structure and build a new one, Nelson explained. In four years of the program, only $1.5 billion worth of preservation work qualified for the credits, Jandl said.

But the Economic Recovery Tax Act of 1981 increased the credit to 25 percent. In each of the past two fiscal years, more than $2 billion worth of rehabilitation work qualified.

Lately, the program has come under renewed scrutiny by Treasury Department officials studying proposals to reform the tax system. However, last month, at a videoconference sponsored by the National Trust for Historic Preservation and the Agriculture Department, President Reagan reaffirmed his support for the tax-credit program.

"With that one initiative, we helped to send your tax dollars back into your communities," Reagan said. He noted that the $6 billion the private sector had spent on rehabilitation work under the program since 1981 was almost as much as "all the taxpayers' money that was spent over nine years during the peak of federal urban renewal."

"We wondered what the fate of the credits would be in the tax reform proposals, and, I guess, now we know in the president's speech," Nelson said. But Jandl was still skeptical: "I'm not sure we really know."

He pointed to the historic Apex Building at 625-633 Pennsylvania Ave. NW as one of the office's successes. Sears World Trade Inc. had planned to save its twin spires but to gut the interior and tear down the building's north wall.

"We told them that we would deny certification, and they quickly halted work to see what could be done," Nelson said. Instead, Nelson said, the wall was strengthened and a spiral staircase, moldings and wallpaper typical of the mid-19th century were saved.

Nelson said developers should apply "before the first brick is touched." "All too often," Jandl added, "they come in too late to save valuable resources."