The National Symphony opens its 55th season tonight with a black-tie premiere sponsored by Neal J. Fox, president of Raleighs. It marks the first time in NSO history that the season's opening concert has had an official sponsor. Special invitations have been extended to the ambassadors of the European countries visited by the NSO on its monthlong 1985 European tour. Among the special guests who are to be seated in the President's Box are Defense Secretary Caspar Weinberger and Jane Weinberger, Chairman of the Joint Chiefs of Staff Adm. William Crowe and Shirley Crowe, Australian Ambassador F. Rawdon Dalrymple and Ross Dalrymple, French Ambassador Emmanuel de Margerie and Helene De Margerie, Anne Petrignani, wife of the Italian ambassador, and British Ambassador Oliver Wright and Margery Wright.

While the NSO was in London last week, Klaus Tennstedt, chief conductor of the London Philharmonic, was ill and the NSO's associate conductor, Andrew Litton, was asked to pinch-hit for him. The 26-year-old conductor apparently was well received. When he arrived home Tuesday, the London Philharmonic was on the phone asking if he would climb aboard the next Concorde and return to London to continue substituting for Tennstedt, who is still ill. Litton had to say no.