The derelicts along Fifth Avenue still nuzzle bottles of Heineken and Blue Nun rather than the cheaper stuff. High school basketball teams still jet off to Las Vegas or Honolulu, and 45,000 homeowners hold mortgage loans subsidized by the state.

Yet as Alaska commuters gingerly guide their automobiles down ice-slick streets on mornings still pitch-black at 9 a.m., it is beginning to dawn on many that the politics of abundance that once lightened the winter gloom here have taken a sudden turn in the wrong direction.

To most Americans, the plummeting price of oil -- to less than $20 a barrel on the latest futures markets -- has brought cheer and good spirits. But, like Mexico, Nigeria and several other foreign countries that depend on petroleum income, this oil-rich state has greeted the change with foreboding.

The change is not unexpected in a state accustomed to an erratic economy, but it has infected a usually youthful, optimistic population with doubts.

Oil companies' profits have been squeezed thin, state oil revenues have dropped $450 million and, for the first time since the precious, hot and thick north slope ooze began filling the pipeline to Valdez in 1977, the state's governor has asked the legislature for a smaller budget this year than the year before.

"Some of the old dreams are going to have to undergo unusual scrutiny," said Scott E. Hawkins, 28, the corporate economist for the Alaska Pacific Bank who finds the Anchorage skyline outside his office suddenly devoid of contruction cranes.

Harold C. Heinze, president of ARCO Alaska Inc., leaned back in his 21st-floor office of the oil giant's gleaming glass-and-aluminum building here and said he thought the petroleum industry would stick with its enormous investment. Oil companies have to plan new ventures years in advance; several new facilities are scheduled to be installed in the next two years.

Then he paused: "I wish I could say with absolute certainty that, whatever happens, I wouldn't change my mind -- but the current situation is very volatile."

For years Alaskans of every race, creed and income have warned each other of the days to come when the oil would no longer flow. But each year the riches from oil royalties and taxes continued to pour in: up to $2.7 billion of the state government's total $3.1 billion in revenues this year. Each year, Alaskans, particularly state legislators, grown famous for discovering their constituents' unmet needs, found ways to spend it.

The oil flow has not abated. The state's oil fields are expected to reach their peak of 1.8 million barrels a day next year. But the dizzying drop in oil prices from the golden days of $36 a barrel appears to have sobered a revenue-drunk legislature to an extent few thought possible.

"There is much more agreement this year to set aside that oil money in recognition of the fact that the peak years of revenues are over," said University of Alaska economist Scott Goldsmith. Anchorage Republican Jan Faiks, chairman of the state senate finance committee, praised Gov. William Sheffield (a Democrat and frequent adversary) for submitting the first budget reduction in the state's history. And Faiks said legislators are analyzing plans that would cut expenditures as much as $200 million.

Even with an extra $2 billion in cash coming to them from various windfall court decisions and tax settlements, many legislators said they are determined to restrain the state's spending habits and add to the $6 billion Permanent Fund designed as a saving account to cushion the thin post-oil years.

Pet projects -- the new dams and railroads and air strips and gymnasiums and docks and community centers and roads and generators that absorbed much of the legislators' attentions -- seem less urgent now. A proposed $6 billion Susitna hydro-electric project 60 miles from here seems unlikely to go forward, despite $100 million already spent in the planning. The Knik Arm Causeway that was going to open up a new area northwest of here for more Anchorage suburban sprawl may have to wait for the next boom, whenever that may be.

The construction industry, after years of such lucrative projects as the Trans-Alaska Pipeline, the Prudhoe Bay oil fields and the creation of modern downtown Anchorage, finds office space going begging, housing starts slowed and even the oil industry cutting back on projects beyond the next two years.

The value of new building permits in Anchorage, where about half the state's population lives or works, dropped 41 percent in 1985 and housing construction dropped 33 percent, Alaska Pacific Bank reported. The construction industry lost 1,500 jobs last year, and another 2,000 jobs are expected to go this year.

For the still-growing cadre of white-collar professionals "it's a little harder to change jobs than it used to be," said Hawkins, the bank economist. "Three years ago, companies would have been pursuing each others' employes a little more aggressively."

Some Alaskans even gave way to temporary panic as the new oil figures came in.

"When these things hit the newspapers, they had a psychological effect," Goldsmith said. "There were some people who said, 'Should we pack our bags and go south?' But that was an inappropriate response."

For the average middle-income worker, the standard of living is so good here that it will take much more than a drop in the price of oil to produce any significant exodus. Residents of the "Lower 48" states have often discounted the high salaries of their Alaskan friends and relatives by noting the higher prices here. Unbeknownst to them, that cost-of-living gap has steadily narrowed.

In 1961, the Alaskan relative price index, as computed by the University of Alaska Institute of Social and Economic Research, was 47 percent above the U.S. consumer price index. By 1982 it was only 26 percent above the U.S. figure.

"We've built warehouses, we have more competition, we have economies of scale now," said Charles L. Logsdon, an economist with the state revenue department.

Goldsmith added that "you don't have to go to Seattle now to buy a nice piece of furniture."

Yet in 1982, per capita incomes here remained 46 percent above the U.S. average. Economists note that the Alaska average would be even higher if it were not for the extremely low income levels of rural residents and some of the lowest paid city dwellers. Homeowners in Anchorage, one institute analysis shows, earn nearly twice the income of the average U.S. homeowner.

In the midst of their worries about falling oil prices, Alaskans do not talk much about these many remaining blessings, but visitors from places like California often are left aghast. The state government spends about five times the national average in per capita services for its constituents, yet charges them no income tax. (This means, Alaskans quickly remind visitors, that their federal tax bills are higher because they cannot deduct a state tax.) The state also pays cash dividends to every state resident from part of the interest on the oil-fed Permanent Fund.

Last year everybody got $404. Although this may seem extravagant to outsiders, Pete Spivey, a special assistant to Gov. Sheffield, argued that it has created widespread popular support for protecting the Permanent Fund from rapacious legislators, just as former Gov. Jay Hammond hoped it would. Sheffield recently returned from about 32 town meetings around the state, Spivey said, and "The message came back clearly that most people are real interested in protecting the Permanent Fund."

Some of the government's oil largess has spread to other industries. The state government has agreed to build a road and dock facilities for the new Red Dog lead and zinc mine near Kotzebue on the northwest coast. Promoters say their new equipment will be able to extract the ore so efficiently and in such large quantities that they will make a profit even with the notoriously low zinc and lead prices.

With some government help, the fishing industry also has rebounded slightly from some bad years. Tourism, fortified by the state's heavy investment in television advertising in the "Lower 48," is expected to improve. Only timber, of the state's major industries, remains sunk in recession.

State economists such as Logsdon note that they had predicted a falloff in oil prices, although not as low as $20, and expect the state to muddle through. A battle of pessimists and optimists continues, with many Alaskans bouncing from one extreme to the other, depending upon their mood.

"If the price drops to $15 a barrel, there could be some fairly dramatic life style changes, people forced to relocate," Logsdon said. "I got to thinking of how much mortgage debt I have in my house. You can kind of scare yourself."

In the rural areas of the southeastern coast and the bulk of the interior outside Anchorage and Fairbanks the potential problems are even greater, for those communities have been far more dependent on the state government's generosity than have the big cities. With timber still moribund and many Alaska native corporations in financial trouble, the minority of Alaskans living in remote villages may find the decline of oil revenues a severe blow.

Oil industry executives such as George Nelson, president of Sohio Alaska Petroleum Co., argue strenuously that many of their troubles can be blamed on over-regulation and over-taxation. Tax and royalty rates of 30 percent may encourage many operators to shut down, if profits decline. A new boost in oil taxes, suggested by some state legislators, could result "in the delay, deferment or cancellation of a lot of projects," Nelson warned.

Max Pitcher, Conoco Inc.'s vice president for North American explorations, said, for example, that he is planning to delay important Alaskan projects because profits seem so uncertain at the suddenly lower oil prices.

Petroleum executives measure their Alaska commitments by sealifts. Each summer the ice near the shore of the North Slope melts for only a few weeks, allowing ships to bring in the heavy equipment for the following year's major construction. No one expects a major downturn in oil activity immediately because ARCO plans a large sealift this summer, and Sohio a large flotilla in the summer of 1987.

"But if you ask me how big the sealift's going to be in 1988," ARCO's Heinze said, "I can tell you: It will be zero."