Amid unusually tight security, a Dutch prostitute and a Washington man told a federal court jury yesterday that they were paid thousands of dollars in the last two years to smuggle high-grade heroin from Amsterdam to Washington in cigarette boxes as well as in girdles and boots they wore.

Testifying on the 13th day of the drug conspiracy trial of accused Washington heroin kingpin Linwood Gray and 11 codefendants, prostitute Helena Visser said that she was paid $20,000 to carry heroin into the United States, but contended that until she was arrested last January she did not know it was illegal because "I'm stupid."

Admitted heroin dealer Ernest A. Fletcher later told the eight-woman, four-man jury that she he was paid $25,000 for several trips to Holland to bring the heroin to the United States and last fall consented to be a government informant about the alleged ring when investigators told him of the evidence they had against him.

Moderate security precautions had been taken throughout the trial.But yesterday everyone entering the courtroom of U.S. District Judge William B. Bryant was required to register and show identification. In addition, lawyers' briefcases were placed in an X-ray machine.

The extra security seemed to stem from the appearance of Fletcher, one of the government's key witnesses, who had been threatened after it became known that he was a government informant. Two other people with alleged connections to the heroin ring have been murdered.

A large, chunky man known in the purported ring as "Fatsy," Fletcher seemed reluctant at first to testify about his alleged dealings with Gray, one of Gray's accused lieutenants, Carl L. (Deke) Cathey Jr., and others. Repeatedly reminded to talk louder, Fletcher eventually testified how he sold heroin on the streets of Washington and willingly made the trips to Amsterdam because he needed the money.

After he was caught, Fletcher said that he agreed to have his house bugged so that investigators could listen in on conversations with members of the alleged Gray ring. The jury is expected to hear testimony today about Fletcher's alleged heroin deals after he became a government informant.

Visser, who is serving a three-year sentence in Holland for possession of heroin in connection with the Gray case, was indicted in the United States, but testified that charges would be dropped against her here.

She said that she became close friends with an alleged Gray accomplice, Robert L. Stuckey, but incurred his wrath last year when he asked her in a trans-Atlantic call to bring him a record album by rock-jazz singer Phoebe Snow. She said she interpreted that as a code for smuggling more "snow," slang for heroin.

But she said Stuckey "acted very strange" and disappointed when she brought him only records and a gift of some after-shave lotion, but no heroin.