The Grosvenor Park Apartment complex in Bethesda was sold last week, but the sale is shrouded in mystery. The buyer refuses to be indentified or reveal the purchase price.

Even some of the tenants at the two stately rental apartment towers and garden apartments at Rockville Pike and Grosvenor Lane in Rockville said they are "up in arms" because they don't know who has bought the building or whether it will be converted to condominiums, as many tenants and others in the real estate field suspect.

"We just don't understand this," one tenant said. "We don't know what's going on here."

Another tenant, who declined to be identified, said she was told the owners are a Greek couple. Still another tenant said she was told the building was sold to a company in chicago.

American Invsco, a real estate holding company in Chicago, arranged the purchase of the complex by a private partnership that does not want to be publicly known, according to an Invsco spokeswoman, Sherry Kamp. "This was not an Invsco purchase," she said.

Kamp said the partnership has made no immediate plans to convert the 629 high-rise apartment units and the 421 garden apartments to condominiums.

However, several persons in the real estate field said the property probably will go condominium within six months to a year. One of the three towers at the complex, which was not sold by the owner, Karl W. Corby Construction Corp., was converted to condominiums in 1973.

According to figures supplied by the Montgomery County Office of Housing, 6,730 rental units have been converted to condominiums in the county since 1974. Of all seven Washington-area suburban jurisdictions in Virginia and Maryland, Montgomery has the highest rate of condominiu conversions, the figures show.

There are 57,000 rental apartment units left in the county, according to Gene Sieminski, office of housing director.

"It's a problem we're having in diminishing rental stock," Sieminski said. Since 1974, the county has "lost more rental units to condominiums than we ever got back" through construction of new apartments.

Because Grosvenor Park complex rents are in the medium- to high-priced range -- up to $700 a month -- possible condominium conversion will not affect low-income families seeking rental units, Sieminski said.

Tenants said they received undated notices in their mailboxes at about 7 p.m. on Jan. 17 that said: "We are pleased to announce that effective Jan. 17, 1979, Grosvenor Park will be managed by the Willo Wick Management Co."

Another undated letter received several days later from the Corby Corp. directed the tenants to pay their rent to the management company, the tenants said.

Spokesmen for the Corby Corp., Willo Wick, American Invsco and the Grosvenor Park apartments said they were forbidden to disclose the name of the owner or the purchase price.