In his spartan fourth-floor walk-up apartment on 14th Street NW, Julio Cesar Hernandez slips a letter and a $350 money order into an envelope to be mailed to a Miami address from which it is later forwarded to El Salvador.

About a week later, his grandmother, 70-year-old Ana Montesinos-Cruz, hears her name on a local radio program from her home in Intipuca, a tiny rural village in southeastern El Salvador. The early morning announcement brings good news: A letter and money are waiting for her at a business in nearby San Miguel.

By nightfall, she has picked up the letter and has traded the money order for 1,750 colones, five times the average monthly wage in that strife-torn Central American country.

For two years, this steady monthly flow of cash from her grandson, a kitchen helper at a Bethesda restaurant, has come to represent survival for Montesinos-Cruz and her husband Ercilio. It also has become part of an unofficial pipeline of at least $350 million and perhaps more than $1 billion that Salvadorans in the United States send back home to their relatives every year.

In recent years, this direct people-to-people aid has become the foreign exchange bedrock of the shaky Salvadoran economy, which for a decade has been battered by a civil war that has dried up most foreign investment, according to U.S. and Salvadoran officials.

The cash pouring into El Salvador has had a noticeable effect on the Salvadoran landscape, in some cases transforming poor, rural villages of straw and adobe huts such as Intipuca into more affluent towns of whitewashed, wood-and-brick houses and paved streets, said officials and Salvadorans living here.

In the case of Intipuca, 12,000 to 15,000 of its residents have settled during the last 20 years in the Washington area, which has about 100,000 Salvadorans, the second-largest concentration, after Los Angeles, in the country, local officials said. The greatest influx of Salvadorans into the United States occurred in the early 1980s as the civil war intensified.

Segundo Montes, a Salvadoran sociologist, recently published a study that said money sent to El Salvador by about 1 million Salvadorans who live in the United States amounts to about $1.4 billion a year -- which, if true, is almost twice the country's 1987 operating budget and about four times the amount of U.S. economic aid to El Salvador last year.

Montes, a well-regarded professor at the Jesuit-run Central American University in San Salvador, based his estimate on detailed questionnaires he gave to 1,300 Salvadorans living in the United States and more than 2,000 residents of El Salvador. His study, completed in August and released late last year, was partly funded by a grant from Georgetown University's Center for Immigration Policy and Refugee Assistance.

Salvadoran officials say Montes' $1.4 billion figure is exaggerated and that a more realistic amount is about $350 million a year, or about $1 million a day.

But even this more conservative estimate represents an essential structural support for the economy of El Salvador, which has a population of 4.5 million and an unemployment rate of 40 percent, officials said.

"If it weren't for this money, El Salvador would have crumbled; it would have fallen apart by now," said a U.S. State Department official who in 1985 conducted a study of the impact of family aid on the Salvadoran economy.

The Salvadoran ambassador to the United States, Ernesto Rivas Gallont, said the family aid helps the country because individual families are able to "invest in construction and purchase consumer items."

So much money is being pumped into El Salvador by Salvadorans in the United States that clever entrepreneurs, many of them Salvadorans, have established an efficient, well-organized system for channeling the money back home.

Across the United States, in cities where large numbers of Salvadorans have settled, dozens of businesses called "transfer houses" have sprung up in the last five years. Operating like express mail services, these businesses for a fee promise person-to-person delivery of the U.S. money in El Salvador.

In Intipuca, a farming village near the Pacific coast, the U.S. wealth is reflected in the town's modest but comfortable homes, many of which have television sets, videocassette recorders and telephones, said officials and Salvadorans living here. It also is a testament to the fact that those who have traveled to el norte have not forgotten their roots.

The town's 5,000 inhabitants, many of them older people and children, are well fed and wear new clothes and shoes, in contrast with the inhabitants of nearby towns.

In the District of Columbia, some of the more successful local Salvadoran businesses are owned by Intipuquenos, including several restaurants and grocery stores, a construction company, a transfer business and an accounting firm.

Last year, about 25 of the more prominent Intipuquenos in this area formed a committee to help their home town. Their $7,000 donation has been used to pave two main streets, paint the local church, build a wall around the town's school and repair the aging clinic.

In the future, the committee plans to send books, pencils, notebooks and T-shirts for the schoolchildren, and medicine for the clinic.

"There is a deep-rooted belief among Intipuquenos that you help the family members who remain behind," said Fernando Leonzo, 38, a wealthy native of Intipuca who lives in Falls Church and remembers well the economic magnet that attracted him to this country in the early 1970s.

He was 18 years old and a university student in El Salvador. His parents had fallen into debt when two trucks they owned were wrecked in accidents. The trucks represented the family's livelihood; Leonzo's father hauled fruits and vegetables throughout Central America for a living.

Leonzo, who has eight younger brothers and sisters, said he overheard his parents talking one night about their economic difficulties, and made up his mind then to go to America.

A friend from Intipuca, who already had traveled to Washington, had told him there was plenty of work in America and that a strong young worker could earn as much as $2.65 an hour -- a significant improvement over the average $6 a day wage in El Salvador.

When Leonzo arrived here, a friend from Intipuca helped him find a place to live: a drafty $10-a-week room in an Adams-Morgan rooming house. Another friend from home helped him land his first job as a $2.35-an-hour dishwasher in a D.C. hotel. Soon he was working a second job at night, in the kitchen at Blackie's House of Beef.

For the first six months, Leonzo sent home $200 a week, keeping for himself only $15 to pay for his rent and food. Back home, his parents were paying off their debts and planning to rebuild their truck hauling business, he said.

As time went by, the amount he needed to send home every month grew smaller and Leonzo gradually moved up to better jobs.

Even after he opened his own business in 1982 with $20,000 in savings, Latin Investment Corp., a firm that in part helps Salvadorans send money to El Salvador, Leonzo continued to work as a busboy at a luxury hotel in Georgetown until 1984. He later established his own construction business here.

He and his family now own several businesses in El Salvador, including restaurants, an appliance store and a gas supply company.

Leonzo, who married a Salvadoran woman in this country, has a 17-year-old son and became a naturalized U.S. citizen last year. He said he has never regretted his decision to leave Intipuca.

"I had promised my parents that someday I would go back and live there," said Leonzo. "But I don't intend to return now, and they have given up insisting."

Ana Bety Reyes, 36, also a native of Intipuca, came to Washington in the early 1970s at the age of 19. She was orphaned at age 12, when her father died suddenly, and was brought up by an uncle in San Miguel.

A cousin in the United States advanced her the money to make the trip here. Her first job was as a live-in, $65-a-week housekeeper in the Green Acres subdivision of Montgomery County. After repaying her cousin the money for the trip, she began sending money home every month to help her uncle and grandmother.

The rest she saved, setting a goal for herself: Once she had accumulated $10,000 to $20,000, she would return to El Salvador and open her own store.

But in the intervening 15 years, Reyes married a Salvadoran man who comes from a village near Intipuca, had four children and opened two restaurants in the District.

As far as returning to El Salvador someday, Reyes, seated at a small table in her Adams-Morgan restaurant, El Tamarindo, said, "I don't know. Maybe someday, when my children have grown. But it isn't anything I think about very often."

Sonia, 24, who was born in San Miguel, said she was 3 when her mother left her and four siblings in El Salvador with her grandmother to go to Washington to work.

For 12 years, Sonia said, she did not see her mother. But each month her mother would send $100 to $150 to help buy corn, rice, beans, shoes, clothing, school books and supplies for the five children.

Then in 1980, Sonia's mother, who had remarried by that time, sent her the $1,400 it cost to pay smugglers to bring her to Washington.

Today, Sonia, who asked that her full name not be used, is married and has two children, ages 6 and 2. She cleans houses for $50 a day; her husband Roberto, a native of Intipuca, earns about $200 a week as a kitchen helper in a downtown restaurant, plus $100 playing the bass guitar at a local disco on weekends.

Between them, they say they send several hundred dollars a month to El Salvador to help relatives: Roberto's elderly mother and an aunt of Sonia's who has seven children and was abandoned by her husband.

"They are poor and they have nothing to sustain them," said Roberto recently, sitting in the living room of the family's sparsely furnished apartment in Adams-Morgan. "They send us letters saying, 'Thank you, this is like rain from heaven.' "

For Julio Cesar Hernandez, 24, the monthly gifts of cash he sends to El Salvador have meant a harsh life of 16-hour workdays, part-time study at the University of the District of Columbia and few possessions with the exception of some books and a typewriter.

During his first six months in the United States, he said, he kept about $130 a month to pay his rent and buy food, sending the rest, about $700 a month, home to Intipuca.

For two years, the money has helped his grandparents, who reared him and his three brothers and sisters, pay off a $3,000 debt on their small farm, and trade a straw-roofed hut for a four-bedroom house with windows, doors and a sturdy roof.

Hernandez now sends about $350 a month to El Salvador, but his living conditions remain spartan because he wants to save as much money as possible to get a college degree in political science and someday buy a house.

For now, he shares a bedroom with a cousin in a two-bedroom apartment. It is a large room with bare windows, two twin beds, a desk and a chair.

His days and weeks are strictly regimented: During the day on Mondays, Wednesdays and Fridays he works in the kitchen of a Bethesda hotel and in the evenings attends English classes at the University of the District of Columbia. On Tuesdays, Thursdays and Saturdays, he works in an Italian restaurant. In his spare hours, he sells encyclopedias door to door. On Sundays, he attends mass at Sacred Heart Catholic Church at 16th Street and Park Road NW and teaches the catechism to Hispanic children.

His busy schedule notwithstanding, Hernandez writes a lengthy letter every two weeks to his grandparents, whom he calls Mami y Papi, Mom and Dad.

"I tell them life here is hard," said Hernandez, sitting at his desk in his sunny room. "I earn a lot, but life here is very expensive. But I also tell them I will never forget them. I will always send them some money."