Rep. Barbara Jordan, the black Democrat from Texas who mesmerized the nation during the Nixon impeachment proceedings with her emotionally voiced belief in the Constitution, said today she will leave Congress at the end of her current term for personal reasons.

While her announcement that she won't seek re-election had been widely predicted, a stunned disbelief greeted her repeated assertion that she has no plans, as yet, for her future. "I don't have a hidden agenda, believe it or not," she said during nine minutes of skeptical questioning. "I really don't."

She said only: "What course I shall pursue in 1979, when my current term expires, I do not know. Whether in the public or private sector, I do not know." And the three-term representative's only explanation was, "The longer you stay in Congress, the more difficult it is to leave Congress. The ability to get out and do something else is minuscule."

"This thing doesn't make sense." said Ben Rayes, state Democratic Party treasurer and a state legislator, part of whose district lies in Jordan's downtown Houston congressional district. "It's not like her to do this. I can't figure it out, man."

Whatever else, Jordan's announcement means that one-fourth of the Texas House delegation - six of 24 - is vacating seats at the end of the term. The unusually heavy, and all Democratic, turnover has inspired Republicans to believe they may be able to wage six or seven genuine congressional campaigns here next year.

A handful of black officeholders already are jockeying for Jordan's seat in a predominantly black and Hispanic district. Jordan said her announcement was designed to allow time for candidates to form organizations before a February filing deadline.

At a press conference in her district office in the federal building here. She repeatedly denied that a U.S. judgeship, a job with one of Houston's many prestigius law firms or anything else was waiting for her. She denied, too, that she suffers a health problem, as some had speculated, and her announcement to "not seek elective office in 1978" appears to rule her out of next year's Senate race.

After 1978, the next statewide races here are Senate and gubernatorial elections in 1982 and at least in the case of the Senate seat, the incumbent is a Democrat, Lloyd M. Bentsen.

In reality then, although Jordan said today. "I've ruled nothing out. I've ruled nothing in," the political options may be few for Jordan, one of those extraordinary women this state seems to rear with disproportionate frequency.

"She has no place to go." Nancy Palm, a prominent Harris County Republican power, said to Jordan's elective future. "She is absolutely at a dead end, except appointive office."

While suggesting that Jordan, who reportedly wanted the Attorney General's job and nothing else in President Carters Cabinet but was turned down, could secure an appointed state judicial post as a step to a federal judgeship or the Supreme Court, Palm raised another possibility:

Jordan could run for mayor of Houston, the nation's fifth-largest city, in 1979 in an election that would occur just 10 months after she completes her congressional term. "If she is going to stay in Houston," said Palm, "I bet that's what she's going to do."

"I would certainly hope," Jordan told her press conference earlier, "that whatever occurs brings me back to Houston."

It was probably a mark of her stature that the explanation by Jordan, who rose to prominence on a perceived sincerity in her eloquent pronouncements could not be accepted as easily as she offered it.

"For reasons predicated totally, and I need to underscore totally on my internal compass directing me to do something different . . . I shall not seek elective office in 1978," she had said.

That did not mean, however, that Barbara Jordan would fall silent on the issues about which she feels so strongly. Asked if she had consulted.