Rho Fleming greets a visitor at the glass-fronted entrance to the Lighting Concepts showroom and leads the way to a small dining room layout. There she uses a long pointer to jab at light switches in the ceiling that magically transform objects from ordinary to dramatic.

Suddenly, lead crystal goblets sparkle with a diamond brilliance. The carpet changes color from warm gray to cool, the textured pattern pops out and recedes. A bouquet of silk flowers goes from chalky white to a creamy hue.

"About a dozen times a day we get designers dashing in here to check out fabric samples against the different kinds of light," Fleming said.

Her firm, a custom-lighting service created by Interstate Electrical Supply of Chantilly, Va., is one of the newest tenants in the Washington Design Center at Fourth and D streets SW, which opened two years ago this spring in a renovated refrigeration warehouse.

Although some of the tenants say user volume has been slow to build, local designers hail the center, with its 117 showrooms, as beneficial to their business; city officials say they regard it as a jewel in their campaign to attract new businesses, and the developer already is discussing plans for an addition.

"I think the center has been extremely successful. In conjunction with the convention center, I think it's the greatest asset to D.C. in a long time," said Dan Webster, president of the Potomac chapter of the American Society of Interior Designers and a partner in Webster-Kirby Design Associates of Silver Spring. "It adds to the whole design consciousness of this area, and goes hand in hand with monumental building here in recent years."

In the past, decorators had to travel in search of products, or shop via catalogues and books of fabric samples. In contrast, the design center offers one-stop shopping, featuring the merchandise lines of more than 500 manufacturers of furnishings for residential and "contract," or commercial, interiors.

The center was planned as a regional trade mart to attract designers and architects from a six-state mid-Atlantic region. That didn't happen immediately. "It's taken people from Baltimore and the surrounding region longer to use the center, but they're coming in more and more," said William H. Field, managing director of the D.C. design center.

According to Webster, the D.C. effort "certainly stacks up equally with centers in major areas like Dallas, Chicago and New York -- surprisingly so for so small a center."

"We're thrilled with Washington. It was virgin territory, such an obvious opportunity," said Philip Kelley, president of Merchandise Mart Properties Inc., which operates the D.C. center as Mart Center D.C. Inc. and also owns the famous Merchandise Mart in Chicago.

"There are about 35 design centers around the country, and I don't know any of those that opened with 95 percent occupancy," he said. The D.C. project is the company's first such venture outside Chicago.

Field said the building is 98 percent occupied. He said a recent survey of tenants indicated that a minimum of about 75,000 square feet would be required to satisfy requests to grow -- an expansion equal to nearly one-fifth of the present building's 420,000 square feet.

In addition, Field said, he has a waiting list of at least 15 firms that have expressed "serious interest" in leasing showroom space once a unit that meets their requirements becomes available.

In late April, Thomas H. Kennedy, general counsel for Merchandise Mart Properties, met with Curtis B. McClinton Jr., D.C. deputy mayor for housing and economic development, to lay the groundwork for possible expansion.

Asked about a projected timetable for expansion, Kelley replied: "We would like to reach a decision within a year."

Tenants of the Washington Design Center by and large share the view that it is a success -- but their enthusiasm is not without reservation.

"It seems to be taking a long time to take off," said Arnold Kradel, an independent furniture representative from Columbia, Md., who has a showroom on the sixth floor.

He added that he is "comfortable" with his "slow, steady rise" in volume. "People just don't give you business immediately," he said.

But for some other tenants, he noted, "My perception is that some feel they would do better in another location. . . . "

Duncan & Huggins, a local decorator fabric shop, opened a showroom in the center in January 1984 out of economic necessity. "We were getting such a heavy volume of traffic that it was imperative we move into the center. We couldn't stay out there on our own," co-owner Sue Perez said of the shop's move from Georgetown after 20 years.

"The first year was rough," Perez admitted, citing high moving and leasing costs. Showroom space in the building rents from $15 to $21 a square foot.

Perez said that, in the end, "There's no question we're happy to be here. . . . I certainly have increased business."

One original tenant of the center that found the going rough was Sheryl-Marin Inc., a California-based designer of fabrics, rattan furniture and decorative pottery. Three months ago, the owner pulled out.

"We found the traffic just wasn't there," said Sheryl Hippard, president of the company, which has showrooms in New York, Miami and Los Angeles.

Hippard said she believes her firm's difficulties here were largely an issue of taste. "We didn't find that people were very interested in what we were doing. . . . We found the area to be very conservative, very traditional," she said, conceding that her firm's line of "highly contemporary" furnishings seems to hold more appeal in the Sunbelt states.

"The numbers are definitely there," Hippard said of the area's professional design community, "but only a small percentage seemed to utilize" the design center. She said she shared the frustration of other tenants who believe the building's owners should do more advertising and promotion.

Among the center's most enthusiastic tenants are a number of high-profile national manufacturers whose sales have mushroomed now that they have a local showroom. Several use the showrooms as regional offices.

"We realized our presence in the design center was an absolute must," said Ron Orendorf, regional manager for Kittinger Co., a leading manufacturer of 18th-century style furniture and the only licensed maker of Williamsburg reproductions. Orendorf said that area sales increased by 25 percent last year and now average 100 units a week.

Steelcase, a manufacturer of state-of-the-art office systems, also has enjoyed a strong increase in sales volume since opening a 6,000-square-foot showroom in the center.

"We had no physical presence here," said assistant regional manager Robert R. Shapland, who worked out of his home for several years.

A spokesman for Steelcase said that since the opening of the D.C. showroom, the company has increased its sales volume in the area by 75 percent, almost double the industry average; in the same period, the company's share of the market increased by 40 percent.

"We see it as 90 percent our responsibility to get people in here, 10 percent the responsibility of the building's management," Shapland said, pointing out that Steelcase has a well-established regional base of customers who are invited into the showroom to survey products. But, "the preponderance of the firms at the D.C. center are residential," he noted. "They need more traffic to do well."

"My perception is that a number of tenants think . . . the traffic is just not what they want it to be," Shapland said.

Part of the problem, some say, is the center's low visibility amid dowdy federal buildings in the sleepy Southwest sector of the District. Yet, generating community excitement about the center is difficult, they acknowledge, because the building is open mainly to members of "the trade" and their clients.

Field and Webster note that seminars are held regularly to generate interest among design professionals and to educate the public on how a design center works. Field also concedes that more publicity may be in order.

For the developer, the D.C. design center "has met our expectations," said company attorney Kennedy. "We wouldn't have gone in there unless we were sure the market existed." A 1981 marketing survey showed the area as an ideal location for a center, both demographically and economically.

Philip Kelley said that in the past two years, Washington has moved up to seventh place from 12th as a market for sales of home furnishings -- and "that changed because of the D.C. design center."

Individual firms lease space in the building, so no comprehensive sales or tax revenue figures are available. However, Kelley said he believes "the minimum impact" in the residential sector alone "is a $50 million increase in buying in Washington," based on per-square-foot sales in the second year of operation.

Kennedy noted that in the late '70s, Kelley was president of Baker, Knapp & Tubbs, a leading furniture manufacturer, when he became convinced of the need for a design center in Washington. So Kelley approached the owners of the Merchandise Mart and encouraged them to pursue the idea of developing a design mart in the District.

Today, Baker is the design center's largest tenant, leasing 23,000 square feet of space.

Kennedy also credits Mayor Marion Barry with an aggressive push to build the center in the District. "We looked at at least 30 locations throughout the area, including Baltimore and Rockville. But Mayor Barry really wanted us in there," Kennedy said. "He kept inviting us back to look at other properties."

Merchandise Mart Properties, a subsidiary of Joseph P. Kennedy Enterprises, bought the Southwest property from the city's Redevelopment Land Agency for $3 million, after the property had sat vacant for two decades. Renovating the old warehouse, building an annex and outfitting showrooms cost an additional $36 million. The result was a post-modernistic box that blends the brick facade of the original warehouse and a glass-tiled annex banded in ribbons of green tile.

If an addition were built, Kelley said, it would occupy 250,000 to 350,000 square feet on part of an adjacent parking lot. He estimates the project would take about 2 1/2 years to construct.

James E. Clay, chairman of the D.C. Redevelopment Land Agency, said that in originally planning the center, the developer informally agreed to abide by the provision of the RLA's "first-source agreement" -- whereby 51 percent of all new hires for a development project are to be D.C. residents -- even before the policy was officially adopted. The company worked with the local Advisory Neighborhood Commission and community groups to get employment referrals, Clay noted.

The building now employs 700, according to Kennedy.

In addition, D.C.'s Clay said that of $16 million in contracts the design center let in 1983 for support services, $7.9 million, or 48 percent, went to minority firms.

For many observers, creation of the Washington Design Center was inevitable -- and long overdue.

The trade center phenomenon began with the opening of the Merchandise Mart in Chicago in 1930, which has grown to include 1,200 tenants and an 833-tenant apparel center next door. The giant trade mart spawned a number of modest imitators beginning in the '50s, but it is in the last decade and a half that the trend has gained momentum.

Today, design centers exist or are springing up in nearly two dozen U.S. cities, including Minneapolis; St. Louis; Portland, Ore.; Seattle; Boston; Grand Rapids, Mich., and Buffalo.

Industry analysts attribute the trend largely to changing patterns in buying and distribution of fine furnishings.

Independent retail sources for middle- and high-end furnishings aren't as available as they once were. And better department stores increasingly are using in-house design staffs to provide customers with personalized decorating services.

Design centers are popular because they can provide two-tier distribution; besides servicing designers, they serve as an extension of retail floor space to offer customers a broader line of products while saving retailers the high costs of floor space and expensive inventories.

Jules C. Rist is a D.C. design center tenant who recently switched course after 31 years in the retail side of the trade because of his perceptions about where the industry is headed.

Last year Rist sold several retail furniture and interior design shops in Northern Virginia and went wholesale at the Washington Design Center -- a step he calls "a quantum leap in this industry." He now operates The Jules Rist Corp. with his stepson, Tim McCuen, offering antiques, one-of-a-kind pieces and top-of-the-line reproductions.

Explaining the switch, Rist said that "in viewing industry trends with regard to the purchasing of quality merchandise, there were two factors that seemed clear to me: First, that general consumer purchase has undergone a deemphasis of price, and second, the elevating profile and visibility of good designers."

With price no longer the overriding factor in selection of fine furnishings, Rist believes the design center concept of doing business -- with its focus on quality and personal expression -- is definitely on the rise.

"I had a longtime retail clientele," Rist concluded, "but over the long range, I felt that this was the way to go."