Nine Girl Scouts and the Maryland Attorney General joined forces yesterday and filed suit against the Girl Scouts' national board of directors to prevent the sale of Rockwood, a rustic 67-acre Girl Scout retreat in Potomac, to a real estate developer.

Rockwood was willed to the Scouts in 1936 by a wealthy Washington woman, Carolyn G. Caughey, for use "as a character building center," according to the class action lawsuit filed in Montgomery County Circuit Court.

The property is worth at least $20,000 an acre, according to observers, and would thus brign a minimum price of $1.3 million if sold to developers. But its sale would deprive all Girl Scouts of its use forever and thus would violate the conditions of Caughey's will, the lawsuit charges.

A spokesman for the Girl Scouts of the United States of America, said he had not seen the lawsuit, but that the board's legal counsel had previously stated that "the sale is authorized, we have a right to sell.

"Our position remains the same," he added.

Tht spokesman, Richard Knox, said the scouts own a 270-acre forested center in New Youk which is used for programs similar to those at Rockwood, and "from an economic standpoint the organization could not maintain facilities that duplicate each other."

But 11-year-old Kendra Moore of Vienna, youngest of the Scouts bringing the lawsuit, did not see it that way. "The board of directors, people who have reached such high positions, should know the Girl Scout law -- 'to protect and improve the world around me,'" Moore said after the suit was filed. "They're not doing too good a job of it."

Moore said she had seen Rockwood briefly one day when some older Scouts were dropped off, and she thought it "incredibly beautiful. I want to go there myself someday," she explained.

A group of adults interested in scouting has been fighting the Rockwood sale ever since they learned about it last summer, according to Ann Pomykala, adviser to a Bethesda troop.

Pomykala said a small committee visited Maryland Attorney General Stephen H. Sachs days after he took office this month, and he agreed to join them in the class action lawsuit.

Sachs said yesterday his office's "sole aim in this suit is to protect the public interest" in the "charitable trust" he believes was set up by Caughey's will. "The public has an interest, long recognized by the legislature, that the integrity of a will which creates a charitable trust... be vindicated," Sach said.

Last year, the board of directors of the Girl Scouts of the United States of America authorized the sale of the 67 Rockwood acres, as well as 26 adjacent acres, Builders Inc. of Rockville, according to the lawsuit.

The builder plans to use it for construction of 180 residential dwellings in the $180,000 to $200,000 price range, the suit stated.

A sales contract has been signed by the Girl Scout corporation and the developer, but the final sale has not been completed, according to Scout spokesman Knox, who refused to disclose the sale price of the property.

Meanwhile, the Potomac wilderness retreat, called the Rockwood Girl Scout National Center, which for years ran camping and nature programs for young Scouts and training courses for adults, and was host to more than 13,000 Scouts in 1977 alone, has been closed, Pomykala said. Pomykala is one of seven adult Scouts. including women from Maryland, Virgomoa. the District of Columbia and Pennsylvania, who joined in bringing the class action lawsuit.