On the surface, little has changed at Mount Vernon. The moss still grows on George Washington's grave, the 18th century plants and herbs flourish and more than 1 million tourists visit there each year.

The Mount Vernon Ladies Association, which bought the place for $200,000 in 1859 and has run it ever since, still refuses the commercial overtures of companies eager to capitalize on the federal heritage. They've said no to Sears Roebuck (which wanted to paint the place), to Detroit auto makers (who wanted to photograph new models in the circular driveway) and to cleaning companies seeking endorsements of mops and sponges.

Admissions have always paid for the $2.5 million annual operating cost, and Mount Vernon receives no federal or state operating funds.

But George Washington's riverside paradise is in financial trouble. Inflation, old age and the high cost of maintenance have hit the 500-acre estate. Costly repairs and improvements have been postponed over the years.

"Certain systems have begun to deteriorate," says Director John Castellani.

The total cost of repairs and improvements is estimated at $6 million. That's more than gate receipts could ever hope to cover, so the Mount Vernon Ladies Association is embarking on a $10-million fund-raising drive -- something they have not done since the 1850s, when the estate was bought from a descendant of Washington.

Treasury Secretary Donald Regan is chairman of the campaign aimed at private and foundation money. The $4 million left after the planned improvements will go to an endowment to defray future operating costs. Mount Vernon has more than a few problems:

* Its antiquated electrical system, installed in 1922 under the direction of Thomas Edison, is beginning to wear. Simple insulated wiring runs through open spaces in the house and must be replaced to meet electrical codes. Cost of repair: $515,000.

* The irrigation system, including a 250,000-gallon reservoir, is at least 40 years old and its corroding underground pipes collapse often, sometimes leaving the 100-member staff and visitors without drinking water. Cost of repair: $100,000.

* Though security problems at Mount Vernon have never been more serious than a trespasser or two, mansion directors want a more modern electronic system. Early one recent morning an ailing security device short-circuited and began smoking. Security men solved the problem quickly, but Castellani fears it could have started a major fire in the mansion. Cost of repair: $280,000.

* George Washington's copious and now delicate papers are confined to a small library where they are exposed to fire hazards and damaging temperature fluctuation. A new library and research center to house them is now under construction. Its cost: about $2.5 million.

Besides it major fund drive, small contributions will be solicited from visitors. But Mount Vernon managers have decided against raising the $3 admission. "That would be counterproductive," says Castellani. Because Mount Vernon receives more visitors each year than any historic home except the White House, it is better off financially than most historic homes. But the high volume of traffic is a mixed blessing.

Upstairs in the Lafayette bedroom, the fine-grained finish on the wooden doorframe has been worn away by the elbows of eager 20th century guests. The graining is authentic, done by a New York craftsman. His work on the doors and frames in the small upstairs hall cost more than $10,000.