It is 50 miles from Washington to White Marsh Mall, but this is the most critical point in the Washington retail market this year.

The Rouse Co.'s newest and biggest shopping center is expected to draw customers away from White Flint and even Tysons Corner, which for years have soaked up business that Baltimore County was unable to absorb.

Now Washington retailers are following the Baltimore shoppers home, building stores in their backyard to capitalize on what is probably the most underdeveloped major market in the country.

"Baltimore has been an understored, undeveloped market," Rouse President Mathias DeVito said at Wednesday's White Marsh opening.

Part of the reason retail development lagged in Baltimore was the difficulty of getting zoning for major suburban shopping centers, he explained. Another was that "the major department stores really have been busy with other markets."

No more. With Washington and its suburbs saturated with stores, Woodward & Lothrop stretched its sales area northward by opening at White Marsh this week and already is planning a second Baltimore store.

Woodies ran head-on into Bambergers, the suburban division of Macy's New York, which has leapfrogged south from New Jersey to Pennsylvania to Maryland.

To meet the two new competitors, the Hecht Co. completely remodeled its store at Golden Ring Mall, a few miles away from White Marsh, and reopened it several weeks before the Rouse project's debut.

Next week Hecht's opens its sixth Baltimore store at Harford Mall in Bel Air, a few miles north and a little east of White Marsh. Hecht's newest store should further strengthen its position as the biggest retailer in the combined Washington-Baltimore market.

It's not just the big names of Washington that are looking north for growth. China Closet from Bethesda and White Flint opened at White Marsh this week. The American Cafe of Georgetown and Capitol Hill fame is booming in Harborplace, the Rouse development on the Baltimore waterfront.

Rouse is taking the lead in the retail resurgence of Baltimore and environs. Harborplace is a mall without an anchor department store, as much an entertainment as a retailing complex. But recreational shopping is an important attraction for the Baltimore Inner Harbor, where last week's opening of the National Aquarium added another draw.

Rouse last month opened a new wing of its Mall in Columbia, the official watershed between the Washington and Baltimore business basins. Sears Roebuck & Co. added a 142,000-square-foot store, and 129,000 square feet of small shops joined Woodies, Hecht's and the smaller tenants already there.

In Baltimore's northwest suburbs, Rouse will open a project called Painters Mill in late 1984 or early 1985, with four department stores and 350,000 square feet of small shops. One will be Hutzler's, the Baltimore department store that also has a branch in White Marsh. Woodies and Bambergers plan to put their second Baltimore-area stores there, and Garfinckel's plans to join the Maryland migration by opening a store in the project.

Rouse's White Marsh illustrates the contrasts between the Baltimore and Washington retail markets.

White Marsh has five major department stores, just like Washington's most recent mall, Fair Oaks in Fairfax County. Like Fair Oaks, it is adjacent to a major interstate highway in a growing outer-suburban neighborhood.

But Fair Oaks opened last fall in a region already saturated with shopping places, and only a handful of its smaller shops were rented when it opened. White Marsh, in contrast, was almost totally leased-up on opening day. The number of vacancies in White Marsh is less than the number of stores that were open at Fair Oaks when it began business.

One factor in the exactly opposite vacancy rates is the leasing strategy of the developers of the two malls. The Taubman Co., owner of Fair Oaks, chooses tenants meticulously and demands top-dollar rents, with no special deals for shops willing to pioneer in a mall that's too big for the market to support. Rouse, in contrast, has allowed several small shops that won't do the volume of business usually demanded of mall tenants -- a health spa, several gift shops and even the local community college.

In contrast to the outright opulence of Fair Oaks' sprawling, free-form mall and soaring skylights, White Marsh has a simple, linear spine lined with 20-foot ficus trees like an indoor street.