During a decade that has seen dramatic economic decline and increased hunger in Africa, few countries have fallen farther or faster than this mineral-rich nation in the heart of the continent.

Zaire's economy and per capita food production have steadily dropped. It has become black Africa's largest debtor even while its flamboyant, authoritarian president, former general Mobutu Sese Seko, has become one of the world's richest men. Each time debts came due, the government negotiated reschedulings and new loans by promising economic reforms -- promises that were broken almost as soon as the new money arrived.

Now, after years of delay and broken commitments, Zaire again has launched a western-dictated program of reforms. This time it has stuck to the program for nearly 18 months, winning new reschedulings and strong words of praise from the United States and other western donors, which have begun increasing their aid. Once again, despite the recent past, the West is lending money to Zaire.

President Reagan last fall expressed "admiration for a country that carries out assiduous efforts to remedy its economic problems." U.S. Ambassador Brandon Grove Jr. says Zaire "is becoming an example for the rest of Africa to follow."

But the price of reform has been steep. For those who live in the slums of Kinshasa, home for most of the capital's 3 million people, the new program so far largely has meant sharply higher prices and increased hunger and unemployment. In rural areas, which the reforms were supposedly designed to revive, the results so far have been decidedly mixed.

Outside official circles, Zaire receives little attention in the United States. Racial violence in South Africa and famine in Ethiopia have overshadowed less dramatic events here. Yet many analysts believe that because of its size, central location and strategic importance, Zaire could hold the key to Africa's future.

It is at once both one of the richest and poorest of nations. It holds half the world's strategically vital cobalt, one-quarter of its industrial diamonds and vast supplies of copper. There is gold in its hills and oil under its seas.

Yet the average annual income in Zaire does not exceed $200, life expectancy for its 32 million people is only 50 years, and infant mortality is among the world's highest.

The chasm between rich and poor is starkly visible. Gombe is Kinshasa's wealthy business and residential district, boasting air conditioned houses, French cuisine and what is reputed to be Africa's largest Mercedes-Benz dealership. Here the government's economic reforms are viewed by many as bitter but necessary medicine. They have caused price increases but have also eliminated import restrictions. One result: fresh Belgian mussels and smoked salmon are again on display.

But a few miles away in the stark concrete-and-mud slums of the Cite, the government's austerity program has had a crippling impact.

A U.S. Agency for International Development report last year characterized hunger here as "serious and extensive," citing a study by the National Nutrition Center that found chronic malnutrition as high as 49 percent and acute malnutrition as high as 12 percent among children under 5 in Kinshasa's poorer neighborhoods.

"Many people are eating only one meal a day," said Kayende, an unemployed university graduate who lives in the Cite with his wife and four children. Those who can afford it, he said, have plain bread and tea in the morning. Others "only eat once in the late afternoon. For the children it is the same."

Zaire's economic decline has followed a familiar African pattern. There were seven years of steady growth between 1967, when Mobutu consolidated his personal power and renamed the nation and its cities, and 1974 when the bottom began to fall out from under copper prices at the same time oil prices began their steep rise.

Like so many other Third World countries, Zaire and its western economic advisers decided to hold tight and wait for copper prices to rise again, borrowing ever increasing sums to tide it over rather than cutting back on government spending and development projects. But copper prices never regained their heights, and Zaire found itself in deep debt.

Then began a seven-year forced courtship between increasingly wary western bankers and a reluctant government, which continued making promises of financial retrenchment in return for new loans -- and continued breaking those promises almost as soon as the money was approved. Four debt-rescheduling agreements were negotiated with private and public lenders between 1975 and 1981, each of which was scrapped before completion when Zaire fell out of compliance, largely through excess public spending.

By 1983 Zaire was sliding toward total economic collapse, beset by bankruptcy, hyperinflation and a $5 billion debt, 80 percent of which was owed to western governments and donors such as the World Bank and the International Monetary Fund. It was then that Mobutu agreed to yet another program of reforms.

At the behest of IMF advisers, he devalued Zaire's paper currency by a massive 500 percent -- meaning that 6 times more local currency was necessary to buy $1 after the devaluation. This brought official rates into line with rates on the thriving black market. Analysts here say the move -- combined with an end to import and foreign currency restrictions and a lifting of most price controls -- in effect legalized much of the black market and cut the incentive for businessmen to smuggle goods in and out of the country.

Mobutu also slashed government spending, announced a series of management reforms to curtail public corruption and abolished or sold off some of the most bloated of Zaire's state-run companies. He also consented to bring in several dozen experts from the World Bank, IMF and other western institutions, who some analysts say now control day-to-day economic management here.

The initial shock of the measures contributed to an inflation rate of 100 percent in 1983. But since then, government economists and their western counterparts say, Zaire has begun a perceptible turnaround. While official statistics are considered suspect, analysts believe inflation came down to below 30 percent last year, while the gross national product showed a slight increase. Zaire was able to meet $300 million in debt repayments, prompting the IMF last month to grant a new stand-by arrangement to loan up to $160 million over the next year to help the country meet its obligations.

Other lenders are expected to follow suit later this year and again reschedule Zaire's debts. Some have even begun making new loans to help rebuild the nation's copper industry and transportation network, albeit for much smaller sums than previously. "No one is really looking at providing funds for 10 year megaprojects," said a European banker here.

To cope with a reduced budget, the government has attempted to streamline its operations. In practice, analysts say, that often means cutting back on bottom-echelon employes more than on the swollen ranks of higher-paid and politically well-connected bureaucrats. Thousands of schoolteachers have lost their jobs, virtually all nonteaching personnel have been fired from the school system, as have hundreds of nurses and other health workers at state-run hospitals.

Workers have received two salary increases totaling around 45 percent, but that sum has been far below the rise in the cost of living. The cost of a bag of cassava, the potato-like root that is the main food staple here, has more than doubled, while the price of gasoline has tripled.

A majority of government workers earn the equivalent of between $30 and $40 per month -- roughly the price of two sacks of cassava, the amount it takes to feed a family of four for the same period. That leaves no money for rent or other necessities. One study cited by the AID report found that even before the massive price increases of 1983, 30 percent of those in the two lowest income brackets spent more money on food than they earned each month.

"Nobody here expects to live on their salary," said an American rural development worker with nearly a decade of experience here. "People start out working for the government with great idealism and high hopes until they finally realize there's no benefit whatsoever for staying idealistic."

When asked how they survive, most Zaireans respond with the verb debrouiller -- to improvise. For most government workers, that means operating on a system of personal gratuities. Teachers charge their students; police at roadblocks collect bribes from motorists; even doctors and nurses at public hospitals expect fees in return for their services above the medical bills charged by the institution.

Doctors' fees range from $2 to $3 for treating minor ailments to $25 to $50 for surgery, according to one Zairean who handles medical bills for a large firm here. "If you don't have the money, it is too bad -- you just have to die," he said.

The cutbacks have also meant a two-thirds increase in public school fees and a 500 percent jump in university tuition. The University of Kinshasa last year cut back its cafeteria for student boarders to one meal a day -- correctly noting that most city residents had been reduced to the same frequency. This year, citing lack of funds, it cut out meals altogether.

Popular reaction to the government's austerity program has been muted. Following student unrest, several universities in the country's interior have been shut down, as has the government's top technical school in Kinshasa. There have been brief and inconclusive wildcat strikes by post office workers, transport employes and teachers.

A series of bombs went off in Kinshasa early last year, killing two persons, and there have been brief flare-ups of unrest in several remote provinces, including what the government alleged was a small-scale invasion of northeastern Shaba province last November by rebels based in neighboring Tanzania. Sources here estimate 150 people in and around the town of Moba were killed by soldiers putting down the revolt, and many of the casualties were said to be civilians caught up in the Army's crackdown.

But generally the public mood seems one of sullen acquiescence. An economics student at the University of Kinshasa explained his silence and that of other students, saying, "Every time we make a protest the university is closed and conditions get worse."