Cigna and Lincoln National Corp. say they will be in the Washington area by the end of the year. Aetna and the Hospital Corp. of America say they're coming, too. And American Medical International is already up and running in the District.

These are among the national insurance and hospital companies that have recently swarmed to enter the HMO business -- the increasingly popular health plans that offer a range of medical services for one prepaid fee.

Among their prime targets is the Washington metropolitan area.

"Every day it seems that another one of the national operators is on the phone or on our doorsteps," said Barry A. Passett, president of the Greater Southeast Community Hospital Foundation and Management Co., which runs a 450-bed hospital in Washington. "It's getting to be a joke around here. It's always the same dance," he said.

"The competition is going to get more heated. The people who are timid of heart or shallow of pocket should not consider entering this area," added David Metz, president of CapitalCare, a Washington-area HMO recently organized by Blue Cross-Blue Shield.

The reason for the interest is simple: The area is full of affluent professionals, especially government workers, just the type that experts say are signing on with HMOs. It doesn't hurt that until recently there was a relative dearth of these plans in the area.

"The Washington marketplace, as far as the demographics of the area, is perfect from our point of view," said Lawrence T. Longacre, senior vice president for finance at U.S. Health Care Systems, a national HMO chain. U.S. Health Care, along with Lincoln National, a life insurer, has slated a July opening date for a Washington-area HMO.

At least a half-dozen other major firms are reported to be either set on, or considering, entering the marketplace in what virtually amounts to a full-scale invasion. As the field gets more crowded, health-care officials predict a price war that could wipe out some of smaller, more thinly capitalized health plans already operating here.

The result, this thinking goes, will be a redrawing of the Washington-area health-care map.

Today, the region still is dominated by the major insurers offering indemnity plans that pay health providers a fee for each service they give people under the coverage. In the future, under this scenario, a few insurers still will dominate, but they will offer a range of products -- from traditional indemnity coverage and HMOs to various forms of hybrids.

HMOs have been in existence since the 1930s, but they began to surge in popularity during the 1970s, as health-care costs soared. HMOs have proved more cost-effective than traditional insurance because they stress preventive care and seek to keep hospital visits, the main cost of health care, to a minimum. By guaranteeing a stream of patients for hospitals, HMOs also have found they can negotiate lower prices for hospitals' services than can traditional insurers.

Nearly 18.9 million Americans were covered by HMOs as of last summer, a 38 percent increase over the 13.7 million who were members at the end of 1983, according to figures compiled by InterStudy, a Minnesota organization that follows the industry. In Washington alone, approximately 450,000 to 500,000 people are enrolled in HMOs, about 14 percent of a population estimated at 3.5 million, and HMO officials and experts say there's ample room for growth.

Much of the recent growth in HMOs, the figures show, have come from the entry of large national firms into the market, and InterStudy reports that the HM0 industry is shifting away from small, independent HMOs to multistate networks linked by common ownership and management. As of June, 38 percent of all HMOs were linked to national firms, compared with 28 percent a year earlier.

The major new players are the insurance and hospital companies, which are diversifying for quite different reasons, according to experts who follow the industry.

For the insurers, getting into HMOs is a defensive measure. They are saying, "If I don't get involved in HMOs or other alternative systems, then my market share is going to erode," said Alan J. Silverstone, regional manager of the Kaiser Foundation Health Plan of the Mid-Atlantic States Inc., which offers one of Washington's largest prepaid options. Hospitals Formed HMOs

The hospital companies, conversely, have watched as utilization of their beds and facilities has dropped precipitously in recent years. For them, setting up HMOs is a way of assuring a steady stream of patients.

The result has been a mass movement in the last few years of industry giants into alternative care -- through setting up their own HMOs, affiliating with existing HMOs and moving into hybrids known as PPOs. PPOs, or preferred provider organizations, allow beneficiaries to use doctors or facilities outside of the prescribed plan, but only at a higher cost to those enrolled.

The firms making the plunge include insurance giants such as Aetna, Cigna and Metropolitan Life, and huge hospital chains such as Hospital Corp. of America (HCA), Humana and American Medical International.

The trend has continued unabated this year. In February, Travelers Corp. paid more than $30 million to buy the HMOs set up by the Whittaker Corp. And last month, the Equitable Life Assurance Society and HCA announced a joint venture to offer HMOs and other health insurance products.

Signs of the national trend can easily be seen in Washington, where up until recently HMO competition has been relatively restrained. Right now, there are eight HMOs in the immediate area, with a few more beyond the suburbs. The dominant players are Group Health Association, an independent, and Kaiser Permanente, which is part of the largest nationwide chain of HMOs.

Recent developments, however, indicate that the period of quiet is over:

*In 1984, HealthPlus, a failing HMO founded in 1979 in Prince George's County, was bought out by Sanus Corp. Health Systems Inc., a holding company for HMOs around the country.

Sanus' owners include McDonnell Douglas and General American Life Insurance Co. HealthPlus is an independent practice association, or IPA, which means that doctors affiliated with the plan treat patients out of their own offices rather than in a central clinic.

*American Medical International, the nation's second-largest hospital chain, last year bought 80 percent of the George Washington University Health Plan. The purchase -- reportedly for $12 million -- was AMI's first of an HMO.

*CapitalCare, a new HMO, was organized last year by the Washington-area Blue Cross-Blue Shield, which is the area's largest seller of traditional health insurance. Around the country, Blue Cross offers approximately 75 HMOs with more than 2 million customers.

*Cigna Healthplan Inc., a subsidiary of Cigna Corp. and the country's largest investor-owned operator of HMOs, offered $40 million to buy MD-IPA, a large Maryland-based independent practice association.

MD-IPA's board approved the sale, but it fell through earlier this year after the company's shareholders, mostly the organization's doctors, rejected the sale. A Cigna official said the company still plans to open an HMO in Washington by the end of the year.

*Partners National Health Plans recently opened a regional office in Washington, and an official says it wants to begin some kind of HMO or PPO in Washington. Partners is a joint venture undertaken last year between Aetna Life Insurance Co. and the Voluntary Hospitals of America, the largest alliance of nonprofit hospitals in the country.

Aetna already is marketing a PPO product in the area called "Aetna Choice," which allows enrollees to pick their physician, but requires them to seek specialty care with prescribed hospitals and doctors.

*Area hospital officials report that they are being inundated by new HMO operators seeking agreements to provide care for prospective enrollees. Other big firms that have contacted the hospitals include: Lincoln National Corp. and U.S. Health Care Systems, which are jointly marketing a product; Travelers; Hospital Corp. of America; and Transamerica Occidental Life Insurance Co. and the Provident Life and Accident Insurance Co., which recently entered into an arrangement along with American Healthcare Systems, a network of nonprofit hospitals.

For the firms already here, the prospective competitors present the most significant -- perhaps deadly -- challenge yet. Most of the newcomers are well stocked with capital, a must in a business where the start-up costs are in the $5 million range and where profits can be hard to come buy in the first few years, experts say. Most are well versed in marketing strategies, another must in the crowded field.

And most say they plan to offer the vaunted "triple option" -- a choice of HMO, PPO or traditional health insurance -- which they believe will attract employers lured by the easy logistics of dealing with one company for all forms of health care.

John Staelin, Sanus' national marketing manager, said the big operators eventually should push out or absorb most of the smaller independents, such as MD-IPA or Physicians' Care, an IPA organized recently by 800 local doctors. While he said there may be niche for a small number, most "have got to merge with or sell out to others, or they're going to go belly-up."

But Gino A. Nalli, MD-IPA's president, contended that the independents have certain advantages over their bigger competitors because health care is still a locally delivered product. Small HMOs Face Stiff Competition

He and other executives acknowledged, however, that they will have to develop better marketing and sales capacities, offer more options and maintain high-quality care in order to compete with the new players.

One official well aware of the increasing pressures is Robert C. Rosenberg, the executive director of Group Health Association, the oldest and perhaps best-known HMO in the area. Group Health doctors recently ended a bitter strike in which the main issues were doctors' work hours and schedules, which HMO administrators sought to increase but which the doctors' union contended would create "assembly-line medicine."

Rosenberg said that the strike reflected the pressures on GHA to make the delivery system more cost-effective. He also said Group Health needs to develop new products, including perhaps a preferred provider arrangement, and pointed out that GHA has generated substantial capital -- $12 million in cash alone -- over the years. But he acknowledged that at some point GHA itself may have to seek an alliance with a larger "deep pocket."

"We've got money set aside for a rainy day, but not a great deal. We could not sustain an arduous price war," he said.