The Pentagon Building, that war-born symbol of military megathink, turns 40 today and, like many another war baby of similar years, is ignoring the anniversary.

No special cakes or ceremonies will celebrate the birthday of the world's largest office building, according to its chief keeper, and certainly no surprise parties. The Pentagon does not like to be surprised.

Along its labyrinthic 17 1/2 miles of corridors, within whose jiffypoured concrete walls, Pentagon legend says, at least one careless construction worker lies entombed, the signs of age are hardly visible.

Though its heating and air conditioning arteries clearly betray the wheeze of middle age, the building itself--like a late-blooming dowager--has probably never looked better. And while it daily absorbs a work force comparable to the total population of Ashtabula, Ohio, Defense Department officials say it should function well for many years to come.

Not everyone, of course, views that prospect with elation.

"They ought to blow the place up and start over," says Marianne Norby, who worked off and on as a technical editor in the Pentagon for 20-odd years before retiring in 1980. "It's the most depressing building I've ever seen."

Norby and the Pentagon's other critics, past and present, see the building as a mind-numbing citadel of sameness: a sort of vast Orwellian anthill that overwhelms and suffocates the human spirit and turns people into drones.

Defense Department officials, somewhat defensively, say it's not that bad. "Most buildings, like most people, have a front, a back and two sides," says Dr. Rudolph Winnacker, retired DOD historian and a 30-year building veteran. "The challenge here is simply to develop a pentagonal mind."

Actually, the building itself remains remarkably efficient. It still has nearly three times the floor space of the Empire State Building and could surround the entire Capitol building in any one of its five corner sections. And though it's close to a mile around the outermost of the building's five concentric rings, no two points in the building lie more than a seven-minute walk apart.

A virtual city with a present population of 23,300 (down from 29,737 during the Vietnam War), the Pentagon has its own hospital, fire department, post office, banks and eateries, plus its own shopping center, gymnasium, subway station and helipad.

It probably functions best, however, as a symbol of the unnavigable vastness of the federal bureaucracy. Deeply enshrined in its lore lies the apocryphal Western Union messenger who lost his way in the building and emerged six months later as a lieutenant general heading a major Air Force weapons system. Some deny that story altogether. Others say he was only a major.

Built in 16 frantic months on the site of a swampy airport, dump and rendering plant, the Pentagon was the stuff of legend and controversy from its inception in July 1941. As the nation drifted closer to the war in Europe, Lt. Gen. Brehon B. Somervell of the U.S. Army Corps of Engineers conceived the notion to consolidate under one roof a mushrooming War Department then scattered among 17 buildings. He ordered the basic plans to be on his desk in three days.

What emerged was a vast structure of five concentric rings containing 5.4 million square feet of floor space. The unique pentagonal shape grew naturally from the site it was originally drawn to fit--a 67-acre tract bounded by five roads just east of Arlington Cemetery.

Somervell promised the building would house 40,000 workers and cost $35 million. When completed Jan. 15, 1943, it cost $83 million and housed just over 30,000 despite the last-minute addition of an extra floor. It had 30 miles of roadways, 21 bridges and overpasses and parking for nearly 10,000 cars. It also had been shifted southeastward to meet the objections of President Franklin Roosevelt, who threatened to veto the entire project.

Construction started Aug. 11, 1941, and accelerated sharply after Pearl Harbor. At its height, some 13,000 men labored 24 hours a day on the building, pouring concrete so fast the builders often outpaced the harried architects.

The structure went up in self-contained sections. The first office workers streamed in April 29, 1942, 7 1/2 months after the groundbreaking and six months ahead of schedule, as construction workers moved on to erect another chunk of the pie.

Congressional critics carped from the first about noncompetitive bidding, inflated land prices and what they saw as a general wedding of grandiosity and waste. But the Pentagon in some respects became the first model of the miracle of American production that ultimately would decide World War II. Somervell went on to organize the nation's worldwide military supply lines. His successor on the Pentagon project, Lt. Gen. Leslie R. Groves, went on to organize production of the atomic bomb.

To the generation conceived in the years just after its completion, however, the Pentagon lives largest today as a symbol not of what America can produce but of what it can destroy.

Deep within the building's subterranean Military Command Center (and the president's electronic reach) sits the starter switch for Armageddon, and scarcely a day passes without at least one antiwar protester demonstrating on the Pentagon concourse, an activity that reached its zenith during the Vietnam War.

Ironically, the demonstrators may have had a role in making the Pentagon a better place to work. The rhetoric of the Vietnam years had so loaded the image of the building that Donald Rumsfeld, secretary of defense briefly in the wake of the Vietnam War, took steps to close the gap between the Defense Department and the general public.

He hung the once color-coded corridors with military art and commemorative displays. He opened a few nonclassified areas to public tours keyed to honoring the past sacrifices of America's fighting men and women, and to the rich role of the military in the nation's history. Today tourists from Oshkosh and New Zealand can buy Pentagon T-shirts in the concourse gift shop.

The world's largest office building, however, remains a unique place to work, encompassing a subculture with its own totems, hierarchies, legends and language. Even the animal life is special. Occasional errant pigeons fly the corridors, dropping in from time to time at the office of an undersecretary. Workers in one Air Force office once pinned a four-inch cockroach to the desk of a chief of building services skeptical of their claims of outsized vermin. The bugs in the basement, one personnel officer claims, "they put saddles on."

"Wall people" emerge from previously unknown office panels after days of tracing the building's miles of steam lines, then vanish again among the water pipes. "Light people" do nothing but change the 200 bulbs that expire each day among the building's 15,000 fixtures. "Code room people" speak their own electronic language. "Secure people" spend days at a time in offices with doors that resemble those on bank vaults.

The mail-carrying tricycles of the 1940s and '50s have given way to electric carts that whisper along the ramps and corridors and stage occasional clandestine races. An Air Force robot mail cart, which tracked a strip of magnetic tape and beeped for attention at each office door, fell victim to cost accounting.

But 40 years after its completion, the Pentagon has survived three wars, a 1972 rest room bombing by the radical Weather Underground, a computer room fire (guards barred Arlington firemen who lacked the proper clearance) and a plumbing failure in 1954 when, as United Press described it, "water flowed in the Pentagon like money."

It has outlasted a 1945 proposal to erect a 22-story office tower atop its five-acre center court. ("Washington's skyline needs something . . . that sticks up," said the Army colonel who floated the idea. "Why, the Washington Monument is nothing but a needle.")

And it has survived what historian Winnacker finds the outstanding feature of the building and the supreme irony of its design:

"From the air," he says, "it makes such a perfect target."