IN THE BUSINESS . . . Sales of existing houses nationwide rose 1 percent in June, according to figures released by the National Association of Realtors this week, a much weaker showing than the 6.2 percent rise posted in May. NAR attributed the slowdown to rising mortgage interest rates, which it said have jumped a full percentage point in the past 60 days. "Obviously, this increase is beginning to take its toll on the housing recovery," said NAR executive vice president Jack Carlson. The association did find that prices rose more in June than in preceding months--the median sale price of an existing home hit $72,300, up $3,100 over May . . . Coldwell Banker Commercial Real Estate has opened a Baltimore office, at the Inner Harbor Center Building. "We recognize that Baltimore is a dynamic, expanding market and we want to be conveniently located . . . ," said resident manager Gary G. Dewey. The firm already has offices in downtown Washington and McLean . . . American University alumni involved in real estate have formed a Washington-area chapter to provide internships, scholarships, assistance to student who want to go into real estate, and programs for members. Bob Moran, of Stern & Moran, is acting president. The group plans a series of seminars beginning in September. . . Goodwill Industries is undertaking a major renovation and expansion of its national executive offices in Bethesda. The project will add 5,662 square feet of office space and bring the building, the Cedarcroft mansion, built in 1924, up to modern safety and access standards. Case Design/Remodeling, Inc. is handling the design and construction . . . David I. Ginsburg has formed D.I.G. Mortgage & Investment Co., a finanical brokerage and mortgage banking firm that will be active in the financing of both residential and commerical real estate. It will be based in Montgomery County . . . Rouse Real Estate Finance has committed $3 million in mortgage money to RJL Associates for homes in its Breconridge Woods subdivision in Fairfax. RJL plans to offer 12 1/2 percent, 30-year fixed-rate loans to buyers of its four- and five-bedroom colonials, priced from $205,000 to $245,000 . . . Prudential Insurance Co. of America has begun construction of Golden Triangle II, the second of six buildings planned for the firm's 37-acre office park on the Beltway between Kenilworth Avenue and Greenbelt Road in Prince George's. The new building will be seven stories tall and contain 133,000 square feet. Prudential expects the entire project to total 900,000 square feet. Meanwhile, Prudential has also annouced its acquisition of eight buildings totaling 474,000 square feet in the Washington Business Park in Lanham. The sellers were two wholly owned subsidiaries of Allstate Insurance Co. and Allstate Life Insurance Co. Prudential now has more than 10 million square feet of commercial space in the Washington area.

PERSONNEL FILE . . . Homart Development Co. has named Wayne Angle to the post of vice president, development, and announced that he will be overseeing the development of Tysons II, the giant multi-use project that Homart is doing as a joint venture with the Lerner Cos. . . . Leigh and Schwab, Inc., has named William O. Vose president of the firm and Robert Grigg senior vice president and sales manager . . . Jonathan L. Kempner has joined Oxford Development, the big multi-family housing developer, as vice president, general counsel and corporate secretary. Kempner has been general counsel and assistant director of the Pennsylvania Avene Development Corp. . . . The Patt Organization, of Baltimore and Pennsylvania, has appointed Mark A. Redding senior vice president in charge of development for the Baltimore-Washington area . . . Ray Middleton has joined B. E. Walton Real Estate, Inc., of Langley Park, as property manager.

ON THE MOVE . . . The Washington Post Co. has bought a three-story, 51,522-square-foot warehouse at 804 First St. SE from the RIS Paper Co. for $1.25 million. Weaver Bros. represented RIS, which at the same time bought four acres at the Hampton Business Park in Largo for a planned 70,000-square-foot headquarters building . . . Atlantic Research Corp. has leased 80,000 square feet in the Virginia 95 Industrial Park in Springfield. The company plans to move its 200-employe Teleproducts Division there from Alexandria. Coldwell Banker and Carey Winston were the brokers in the deal.