U.S. District Judge Thomas Penfield Jackson ordered the U.S. Marshals Service yesterday to arrest a key prosecution witness in the drug conspiracy trial of D.C. Mayor Marion Barry and bring her to Washington, possibly to be examined by doctors and to testify.

The witness, former Barry girlfriend Bettye Smith, has been in a hospital in Chattanooga, Tenn., complaining of stress. Prosecutors, who have called Smith a reluctant witness, obtained a court order from Jackson last week ordering her to testify.

Smith's unavailability last week caused Jackson to declare a recess in the trial early Friday afternoon, to give Smith time to get to Washington by this morning.

Prosecutors consider Smith an important witness because she has told a grand jury about drug use with the mayor on key dates, including last January, shortly before Barry was arrested in an FBI sting at the Vista Hotel. One count against the mayor, alleging cocaine possession between New Year's Day 1990 and the day of the Vista sting, Jan. 18, was based on Smith's grand jury testimony.

If she does not testify in person, the government told the judge yesterday it wants to introduce her grand jury testimony into evidence. Barry's lawyers have objected to that tactic.

If she appears and does not testify against Barry in the way that prosecutors thought that she would, prosecutors can confront her, while she is on the witness stand, with her previous grand jury testimony.

On Thursday, Jackson issued a bench warrant for Smith's arrest, but stayed its execution based on promises by Smith's lawyers that she would be in Washington by today, according to a source.

But in a hastily called hearing late yesterday afternoon, Assistant U.S. Attorney Judith E. Retchin told the judge that she had received a telephone call from Smith's attorneys about 3 p.m., indicating that they wanted to ask for another delay.

One of Smith's attorneys, Paul Friedman, told the judge that the situation he described last week "has deteriorated," and that Smith would be unable to be in court today. Although Smith's doctor had declared her fit to travel, Friedman said "he sees a very serious risk to her being brought here and forced to testify."

The government's case against Barry, he added, "could survive without her testimony." But his arguments failed to persuade Jackson.

"I have a suspicion, and it is no more than a suspicion, that I'm being played with," the judge said.

When Friedman told Jackson that his client might be able to travel if relatives and friends accompany her, the judge responded, "friends and family may not be available . . . but the U.S. Marshals Service is."

Jackson lifted his stay on the arrest warrant and ordered that federal marshals go to Chattanooga to bring her to the District. He said one of her lawyers, William Currier, could accompany her.

Friedman had said that Currier was on his way to Chattanooga yesterday to try to persuade Smith to testify voluntarily. He was still at the hospital late last night.

A U.S. magistrate also visited the hospital last night. The magistrate, John Y. Powers, set an identity hearing at 6 a.m. today in U.S. District Court in Chattanooga, and the Marshals Service chartered a 6:45 a.m. flight to Washington.