The thing about Washington that J. Willard Marriott noticed most when he passed through in the mid 1920s was the hot summers. Root beer, though Marriott, will go well with all the heat.

So, 50 years ago yesterday morning, Marriott opened a nine-seat root beer stand in the 3100 block of 14th Street NW and began selling cold mugs at a nickel apiece to hot and thirsty Washingtonians. Marriott had anticipated well.

The company now employs 13,600 people in the Washington area - Washington's largest private employer - and its 60,000 employees around the world severe an average of 1 million customers a kitchens, two amusement parks, three cruise ships and a world travel service. Facilities are located in 39 states and 18 foreign countries.

About 5,000 persons gathered in a small meadow near Montgomery Mail yesterday as Marriott, with a dash of American patriotism common to almost all Marriott ventures, nostalgia, live entertainment, balloons and a picnic lunch for all, honored its past and vowed continued growth. Also included was official groundbreaking ceremoniesuarters complex in suburban Bethesda.

Looking out over a sun-baked audience of Marriott employees, stockholders, firends and many of their children, Sen. Charles McC. Mathias (R-Md.) called the event "an extremely important anniversary." The fact that Marriott Corp, could grow from a nickel root beer stand to its present status, "is proof of the vitality that exists in America . . . its free enterprise system . . . proof we can still do it here if we do it the right way," Mathias said.

Founder Marriott, now 76, also spoke briefly and recalled how his own business has proposed with growing use by Americans of automobiles. A devotee of hard work, Marriott's parting words to the audience were typical. "Take care of our customers."

While studying at the University of Utah in the mid-1920s, Marriott became intrigued with a root beer stand there and its daily business volume. He passed through Washington one year and concluded: "I knew a cold root beer would be an instant seller."

He appproached the A&W Co., purchased franchise rights for the Washington, Baltimore and Richmond areas, and came east with $3,000 - $1,500 earned by selling underwear and the balance from a loan from a bank in Utah.

Shortly after opening his 14th Street stand, he went back to Salt Lake City to marry Alice Sheets and brought her to Washington in a model T Ford, a trip that took 11 days. She has been a partner in the firm ever since and now is a vice president and director of Marriott Corp.

The Marriott's will celebrate their 50th wedding anniversary June 9 and in a demostration of the company's 50-year emphasis on show business type promotion to boost its business, J. Willard and Alice Marriott were introduced to yesterday's throng as they role up a small hill to the groundbreaking in a 1927 "Tin Lizzie," as Marriott called it.

No business enterprise started in metropolitan Washington has been quite the American success story that Marriott Corp. now represents.

J. Willard (Bill) Marriott Jr. one of the founders' two sons and now president of the company said in a moment reflection yesterday that he is convinced any other American could duplicate the success story.

"There is plenty of opportunity in American if you work hard, sacrifice and pay attention to the business," he said. While fresh ideas are important, Marriott emphasized a belief that has been obvious from his familys activity: "There is no substitute for hard work."

Marriott and his father have been described by associates as truly enjoying work more than play. And the two men have been the dominant figures in the company's business development - Marriott Sr. by expanding during the Great Depression to such fields as drive-in restaurants and airline catering when other firms retrenched, and Marriott Jr. by vastly increasing the scope of the company's empire since he took over as president in 1964.

When Bill Marriott was named president 12 years ago, Marriott sales were $85 million a year. Sales for the current fiscal year, ending in July, will top $1 billion for the first time.

When Washingtons stop by a Hot Shoppes Cafeteria, a Big Boy or Roy Rogers Restaurant, a Farrell's Ice Cream Parlor or such dinner houses as Hogat's Phineas, Josha Tree and Garibaldi's they are eating Marriott food.

Marriott's total investment in the District of Columbia and five nearby countries of Maryland and Northern Virginia is $87 million, including four hotels, two airliner catering kitchens, 100 restaurants, 42 food service management accounts for such customers as the World Bank and Children's Hospital and the Fairfield Farm Kitchens, a central complec for preparation of many food products.

Marriott first sold its stock to the public in 1953 (under the name Hot Shoppe) and currently is traded on the New York Stock Exchange.

Although Marriott has never paid a cash dividend to investors, it periodically distributes stock dividends and was characterized by Wall Street brokers and recent years as "growth stock."

Some brokers have based recent concern on Marriott's entry into the amusement park business, which involved record capital expenditures of $300 million in the past two years to open parks near San Francisco and Chicago.

The success of the two parks has far outstanced Marriott's own projections, however, and the parks were expanded before opening for the 1977 season. A third Marriott park is planned in the Washington area, either between Washington and Baltimore or near Manassas.

Marriott profits declined a bit with the quarter that ended last Feb. 11, partly because of cold winter weather, but company president Marriott said in an interview yesterday that his firm soon will report "a very good" quarter for the three months that ended in May, "better than we hoped."

In fiscal 1976 Marriott had profits of $30.8 million, a new record, and 41 per cent higher than the previous year. The company's average growth in profitability has been 20 per cent annually for the past decade.

A key ingredient in Marriott's success has been to follow developing lifestyles and to alter his business to meet new trends. Most of the original Hot Shoppes, the foundation for the current corporation, have disappeared in recent years because most Americans now want to eat at fast food outlets or in more sophisticated dinner houses with limited menus.

The founding Marriotts have been prominent in the Republican party. ChairmanMarriott Sr. was a strong backer of former President Nixon and was selected to chair both the 1969 and 1973 inaugural ceremonies.

Marriott also is executive chairman of the Honor America Committee, a national organization to promote patriotism and has been president of the Washington Stake (diocese) of the Mormon Church for 9 1/2 years. His charities include a library in his name at the University of Utah and the Marriott Activity Center at Brigham Young University.

Alice Marriott has been a member of the Republican National Committee since 1959 and she was treasurer of the Republican National Convention in 1964, 1968 and 1972. She is active in many Washington area cultural and civic activities including the Kennedy Center, where she serves as a member of the board and its executive committee.

Son Bill Marriott, who succeeded his father as chief executive in 1972, has avoided the active political identification that his father had. He is the director of the U.S. Chamber of Commerce and a member of the governing board of St. Albans School in Washington, from which he graduated.

The 34-acre site where Marriott is constructing its new headquarters is located at Interstate 270 and Democracy Boulevard and the proposed seven-story building will house about 1,400 employees when completed in 1978. A 1,500-car parking lot will be located on landscaped grounds.

A cornerstone unveiled yesterday dedicates the building as "the J. Willard and Alice S. Marriott building." After the cornerstone was uncovered and the ground broken, several hundred employees grouped together yesterday to spell out the message "Happy 50th." The company announced that it would establish a special garden at the headquarters as a gift to its employees.