From the Kingdom of Saudi Arabia to the American business community, this message:

Saudi Arabia's spectacular oil-fueled boom of the 1970s is ending. The country now has a decelerating, maturing economy, and is running a substantial budget deficit. The big-ticket items, the airports and petrochemical plants and power stations, are on line or nearing completion. Revenues and spending are declining. But this environment offers new opportunities for American business investment, and the Saudis are anxious to expand trade and investment links.

"The days of the massive windfall are definitely ending," Abdullah Alireza, one of Saudi Arabia's most prominent merchant princes, told some 400 American business executives at a conference here last week. "They have been replaced by a cautious, discriminating and selective approach."

Potential newcomers to the Saudi marketplace, he said, "will be up against some of the fiercest competition in the world. But there are good profits to be made and good contracts to be won by those who pursue the Saudi Arabian market in the right spirit. For those who stay the course, the reward will be worth the struggle."

He and other Saudi officials and business executives said that the kingdom's needs, and its tastes, are changing now that the basic infrastructure of industry and communications is well-developed and the buy-anything surge of consumer spending is cooling off.

This means, they said, that smaller American companies--the ones not in the same league with the Bechtels and Mobils and Westinghouses that undertook the giant projects of the past decade--can carve out a niche for themselves providing goods and services not previously known or needed.

But the Saudis also criticized the Americans for what they said is a lack of energy and initiative in the Saudi market, and Alireza shook up the conference with a stinging lecture on the inadequacy of U.S. sales efforts.

At the conference, sponsored by the U.S. Chamber of Commerce and the Saudi Gazette, an English-language newspaper in Jeddah, Saudi officials and Americans based in Saudi Arabia cited stark statistics to show how dramatically the oil boom has slowed and the country's needs are changing.

The national budget, which was $91.4 billion in 1982-'83, has been pared to $75.8 billion this year, and is expected to show a deficit of at least $10 billion--more if oil prices continue to decline. The $8.1 billion budget for "manpower development," or training and education, is greater than the budget for transportation and communications.

In its trade with the United States, Saudi Arabia actually had a deficit of more than $1.5 billion last year, as the dollar value of its oil exports to this country dropped by nearly half, from $14.4 billion to $7.4 billion, and imports from the United States rose sharply. Saudi Arabia is approaching self-sufficiency in several basic foodstuffs, including poultry and milk. And the non-oil sector of the Saudi economy, virtually nonexistent a decade ago, has been growing by 14 percent a year, and is now larger by itself than the entire national economy of Denmark.

As a result, "the old days of the large, ready-made turnkey projects are ending," said Bakr Abdullah Bakr, rector of the University of Petroleum and Minerals in Dhahran. What Saudi Arabia needs now, he said, is help in developing technology and trained manpower in "education, food production, food processing and handling, minerals exploration, water and its environment and permanent energy sources."

He said Saudi Arabia, though no longer primitive, presents "a unique environment calling for unique solutions," including technological innovation in such fields as building materials and Arabic data processing, health care, security systems and irrigation equipment.

He and other Saudis stressed, however, that the kingdom is interested in doing business only through joint ventures that give Saudi participants both technological training and management authority and that enable Saudi Arabia to produce its own industrial commodities."

"Saudi industry and private enterprise ask you to help Saudi Arabia be a dynamic factor in your corporation, not a just a flag-waver to say you have a Saudi participant," said Faisal Bashir, a director of the Saudi Basic Industries Corp. "It is in the long-term interest of the corporation to have Saudi executives--not janitors, not drivers."

The lure of the still-potent Saudi market attracted a wide variety of American corporations and sales managers--most of them not previously involved with Saudi Arabia--to the conference, looking for a way to cash in. Among them were:

Joe Emory, president of Float-Away Products Inc., of Atlanta, a manufacturer of metal closet doors and shelving. He said he had never sold anything to Saudi Arabia until recently, when a Saudi subcontractor on a Bechtel construction project "called me up out of the blue and ordered two container loads."

Eldon C. Muehling, sales manager of Pure Water Inc., of Lincoln, Neb., a maker of home water distillers. He said he discovered on a visit to the United Arab Emirates, next door to Saudi Arabia, that "everybody was drinking bottled mineral water," all imported. "They were paying $4 a gallon for pure water. We can make it for 12 cents a gallon," he said. Saudi Arabia "should be a hot market."

J. Chandler Peterson, president of Peterson Wealth Management Inc., and a director of the Saudi-owned National Bank of Georgia. Peterson said wealthy Saudis are looking for "stable, actuarially predictable, safe long-term investments," and he has just the thing: Georgia timber land.

Robert Kern, of Polycoat Systems Inc., Hudson Falls, N.Y., a maker of industrial coatings and polyurethane foam insulation. "They need insulation, and we sell the chemicals for spraying it," he said. "Just the King Khaled Military College they're building, it's millions of square feet, and it will all be polyurethane insulated. We know the sheikh who dominates this business over there. He already has his ties, but we want to see if it can be spread around."

The Saudis and several Americans with extensive experience in Saudi Arabia made clear to the neophytes that making money in Saudi Arabia requires more than exchanging business cards at a conference in Atlanta.

Saudi Arabia "is still a good place to do business," said Edward Kelley, a vice president of the consulting firm of Booz Allen & Hamilton, but "you have to be prepared to compete in an atmosphere made more rigorous by growth deceleration." Prospective newcomers, he said, must select the right Saudi partner, research the market, send employes who have a long-term commitment and "stay out of marginal, no-growth industries."

Alireza, in the most-applauded presentation of the two-day event, went further. As a Saudi who has a graduate degree from Georgetown University and a long association with the United States, he said, "I am deeply disturbed at the steady decline of the American share of the market in Saudi Arabia, and the widening U.S. trade deficit."

Saudi Arabia, he said, has "a stake in your economy, and to us it appears that the United States has lost a considerable portion of its will to export. Fewer and fewer American products appear in our infrastructure and in our retail stores," he said, and despite the overall growth of Saudi imports from America, U.S. producers now have only 13 percent of the consumer market.

"American products used to be synonymous with quality and American contractors were preferred, but this is far less true today," Alireza said. "We have a healthy economy, far more diverse than when we began, but the competition is very severe. We see your door-to-door salesmen, with their winning ways and high quality products, far less frequently than in the past. Where has he gone? In his place we see the hustling Japanese.