"The Mountains" - as the Catskills are lovingly referred to by New York City dwellers - may be on the verge of a new era. This verdant hilly land where the legendary Rip Van Winkle napped has been catering to vacationers for a century. But with the inauguration of scheduled airline service it is now easily accessible for the first time from every part of the United States.

Allegheny Commuter Airlines started the service to Sullivan County International Airport this spring and the results have been "astounding," says Ben Kaplan, the county's public information director. Sullivan County and Delaware, Greene and Ulster Counties make up the Catskills region.

"The Catskills are winning a whole new audience," said Kaplan, "not only from the United States but Europe as well." Kaplan said vacationers have been flying in from as far away as California and Colorado, but most of the new guests are coming from Miami, Atlanta, Philadelphia, Baltimore, Washington, Cleveland, Detroit and Chicago. "We're even getting visitors from England, Germany and Spain."

Joel Dowshen, spokesman for Allegheny Commuter, said the airline has several daily flights to the Catskills connecting with continental and international flights at New York's Kennedy International, Newark International and Philadelphia International Airports. They are considering expanding the direct flight service to Washington National Airport. There are now daily direct flights from Washington via Philadelphia and New York, and connecting return flights. The airline uses 27-passenger Nord 262 turbojet aircraft.

Because of the new air service, and because casino gambling appears to be on the horizon, optimism runs high about the resort area's outlook.

"With the success of casino gambling in Altantic City, it's just a matter of time before we have it here. We have to remain competitive," said one resort operator. He said he expects to see a casino gambling bill passed by the state legislature before the current session ends. However, even with passage this year, the gaming tables could not go into operation before 1980, under New York State law.

Increased air transport and the vision of casinos has caused major resort owners to bet heavily on the future - over $20 million so far this year on building and refurbishing.

The Concord, which claims to be the nation's largest vacation-convention resort, recently opened an 11-story tower with 200 luxurious rooms and a rooftop restaurant. The resort, which has 1,200 rooms, also has a mammoth exhibition hall that could easily be converted into a casino for legalized gaming. Among the other hotels in the area adding new facilities are Brown's, which built a 78-room addition; Stevensville, which added 32 guest rooms, and the Pines, which put in new indoor tennis facilities.

The vacation business in the Catskills got rolling in the late 1800s with the building of the Ontario & Western Railroad, which linked New York City with the upstate region. During the summer, farms and boarding houses opened their doors to visitors, many of whom came to escape the sweatshops and steaming August sidewalks of New York, less than 100 miles away.

The railroad vanished, to be replaced by modern highways and now by air service. And many of the 19th-century farms and boarding houses also vanished, transformed into plush resorts covering hundreds of acres.

Through the years the resort area, though it had its up and downs - especially during World War II and the advent of the jet age 20 years ago. Business was less than great during the war years of the 1940s because of gas rationing and the lack of eligible males. Singles working girls from the metropolitan areas used to arrive by the busload, only to find more single women. The resorts weathered the war by offering special low rates to single men and GIs home on leave to keep up the sexual balance.

With jets and cheaper air fares, many Catskills regulars began to opt for more exotic destinations. Some of the hotels were forced to close, while others poured in huge sums to improve facilities. Indoor swimming pools, ice skating rinks and tennis courts were built. Health clubs were installed. And for families, supervised day camps, along with night patrols and baby sitting programs were set up.

The projects paid off. Not only did the Catskills regulars return, but new vacationers were attracted. With them the ethnic makeup of many of the resort's clientele also began to change. At one time almost totally Jewish, the area began to take on a universal flavor. Though many hotels still observe dietary laws, their guests are becoming increasingly a cross section of the American vacation mix.

The region has also taken on an international flavor with some hotels featuring German, Hungarian, Irish, Italian, Russian and other national menus and entertainment. There are even two Transcendental Meditation hotels run by the Student International Meditation Society.

Today, the year-round Catskills resorts cater to more than a million visitors. And though they've keep with the times, some things have not changed - chiefly the all-inclusive plan, which they originated. Under the plan, everything but tips, bar bills and sometimes special fees for golf, horse-back riding, etc., are included in the price of the accommodations. So guests can eat, be entertained, enjoy the facilities and know in advance exactly what it will cost.

Available are some of the most elaborate menus found anywhere (Concord guests have an option of 120 selections during a normal three-meal day). Top entertainers appearing in the Catskills this season include Roberta Peters, Ray Charles, Dom Delouise, Jerry Lewis, Totie Fields, Bill Cosby, Robert Merrill and Myron Cohen.

Another thing that hasn't changed is that just about every hotel is family owned and operated, which adds up to more personalized service. And the accent on sports continues. For spectators, there's Monticello Raceway, the state's third largest pari-mutuel harness track. For participants, just about every activity (Sullivan County alone boasts 19 golf courses, many right on resort grounds).

Weekly summer rates at the top resorts range from $215 to $400 per person. They all also offer lower mid-week rates as well as golf and tennis packages. The rates come down after Labor Day. The region also abounds with lower-priced cottage colonies and tourist homes, many with swimming pools and entertainment.

This year, New York State has designated the Catskills region the site of the state's "I Love New York" Fall Festival. Many events are planned, including fall foliage tours, bicycle weekends, concerts and parades.

Information is available from Ben Kaplan, director, Sullivan County Public Information Office, County Government Center, Monticello, N.Y. 121701.