"So what are you going to do? Are you buying, or are you moving?"

The rotund, elderly man with the thin white mustache was standing in the lobby of the grosvenor Park apartments in north Bethesda, talking to a well dressed, elderly woman. Both are facing critical decisions. Beginning next month, two-thirds of the 1,500 apartments in the Rockville Pike complex will be sold as condominiums.

"I don't know what to do," the woman answered. "My husband's in the hospital and I don't know if I'm coming or going. Now they've got to bother me with this."

In the lobby, the elevators, the laundry room, the small Grosvenor grocery store, the topic is always the same these days. Grosvenor tenants are joining hundreds of other Washington-area residents in facing a collective trauma: What do you do when your apartment complex goes condominium?

Most tenants are angry, frustrated and, frankly, afraid that they won't be able to find affordable housing in Montgomery County.

"You feel that they've taking everything from under you. You feel uprooted . . . You suddenly become aware that nothing is certain," said one resident, a saleswoman at Saks in Bethesda, who decided to buy her unit.

"You know, you can handle just about anything between mine to five. But at the end of the day you'd like to know you have a place to come home to," she added.

In January, the 18-year-old complex was sold to a firm called Grosvenor Inc. for $30.4 million. Two weeks ago, Grosvenor residents learned that the firm, a subsidiary of American Invsco of Chicago, was planning to convert two-thirds of the complex to condominiums.

A total of 1,050 apartments are going on sale - 629 in the three highrise buildings of the complex, and 421 garden apartments.

To persuade the current tenants to buy, the new owners are promising to pay them a total of six months' rent on their units and two months' worth of condominium fees from the complex's reserve fund. The condominium fee payments alone would range from $140 to $1,028.

The cost of the condominiums will go from $30,000 for an efficiency to $250,000 for a luxury penthouse with five bedrooms, a library, and a fireplace.

But no matter what sweeteners have been offered them, many Grosvenor tenants still seem to feel that they are living in an "occupied city," as one resident put it.

To the tenants, the "invaders" are the representatives of the owners and the management company who walk around the buildings wearing lapel pins identifying their company. The "besieged" are tenants who glare at the newcomers as they show prospective buyers around the building.

"When I walk in (the lobby) and see that (condominium) sign, I just feel so angry," said 28-year-old Debbie Fine, a civilian who works for the county police department.

"It's a strange thing not to feel you have a home."

As she speaks, the sights and sounds of condominium conversion are evident: Workmen are plaster-patching and repainting the hallways near the condominium sales office and putting in new hallway light fixtures. A moving van parked in back of one building is loading a resident's belongings.

The tug-of-war between the Grosvenor tenants and the new owners is both a philosophical and economic one.

Tenants complain that people who can't afford to buy property - or don't want to - are virtually being squeezed out of the area. In Montgomery County alone, 6,730 rental units have been converted to condomniums since 1974. That's more conversions than in any other area jurisdiction.

The businessmen doing the converting argue that they are offering tenants a tax shelter and equity in lieu of a pile of virtually worthless rent receipts.

"We're a human company," said one representative of Home Marketing of America, Inc. - the sales and marketing arm of American Invsco - as he cited the various inducements being offered to the tenants.

But those arguments have done little to ease 75-year-old John Larue's mind.

Larue, a widower (not his real name), lives in a one-bedroom apartment that has been renting for $332 a month.

Living on $818 a month, Larue, a slight man with a shock of the whitest kind of white hair, says to purchase the condominium for $57,900 would be "out of the question." His monthly payments would be in the $510 to $628 range, according to a broker for Grosvenor.

"I don't know how I'd go to the grocery store," says Larue, pulling out a $1.35 package of chicken bologna from his refrigerator. A month ago, it was $1.19, Larue says, shaking his head.

But if he dips into his savings, Larue says, "maybe, just maybe I'll be able to swing something in the $30,000 range."

Larue says just the thought of packing and moving is causing him sleepless nights and nightmares. "When I think about moving, I just about lose my mind. I don't know if I'm physically able," he says, resting his pale face in one hand.

Larue, a retired Internal Revenue Service employe, has looked into moving to the Willoughby in Chevy Chase. "But here's the thing, that apartment could go condominium in a couple of years." Laure says that, at his age, he could not tolerate two moves.

One time, just one, Larue says he cried as he described his thoughts to his sister.

Deborah Fine is 27, single, and making more than $15,000 a year. Although her circumstances are quite different from Larue's, she feels many of the same emotions.

She feels she can afford to buy her efficiency at $34,500-and could benefit from the income tax break and equity. But she isn't sure she wants to make that big a financial commitment at this stage in her life.

"I enjoy going out to lunch and dinner. I enjoy taking vacations, going to concerts and theater, shopping, and these are the things I'd be trading off. . . . You may get the money back at the end of the year, but you've got to live during the year."

Fine is paying $254 for rent. Her monthly payment, if she makes a 20 percent down payment, on her apartment would be about $395, she's estimated.

Fine says the condo conversion at Grosvenor has spurred her to take a look at her life in general. And, like Larue, she is torn. "On top of it all, you're constantly being influenced by people who say, 'This is the greatest (opportunity) in Washington. If you put your money in a home, you'll never regret it.'

"All these things are emotionally rattling because it's like you're afraid you're passing up a good deal."

Dr. John Kalberer is 43, a senior scientist at the National Institutes of Health. His wife, 37, is a senior personnel officer there. They have two children.

Kalberer says his and his wife's combined salaries have moved into the $80,000 range since he first came to live at Grosvenor. So instead of buying their three-bedroom apartment with three bathrooms for $105,000 he bought a $150,000 townhouse nearby.

"We got cut-rate prices at Grosvenor," said Kalberer, whose unit was renting for $522. "In essence, we were spoiled. . . ."

Spokesmen for Grosvenor's new owners make the same argument. They maintain that Grosvenor's former owner, Karl Corby Jr., and the county's rent control laws kept the rents unusually low.

By comparison, efficiencies at the nearby Pavillion apartments start at $280, while the Grosvenor's started at $228. But a three-bedroom apartment at the Pavillion ranges between $475 and $535, while Grosvenor's three bedrooms and den units ranged between $585 and 1,220.

"I guarantee most people will gulp . . . when they find out what rentals are going for in this area and they'll run back" to buy their apartment, said a representative for Rockville Grosvenor. CAPTION: Picture, At Grosvenor Park, building at left is already condominium; two others are undergoing conversion now. By James M. Thresher-The Washington Ppost