A federal jury, with some members in tears, took 6 1/2 hours today to find former Green Beret doctor Jeffrey R. MacDonald guilty of murdering his pregnant wife and two young daughters Feb. 17, 1970.

MacDonald, 35, an emergency room physician in Long Beach, Calif., sat immobile as the jurors, four of whom were crying, were polled and pronounced him guilty of second-degree murder in the slayings of his wife, Collette, 26, and daughter, Kimberly, 5. The jury found MacDonald guilty of first-degree murder in the stabbing of his younger daughter, Kristen, 2.

U.S. District Court Judge Franklin T. Dupree Jr. sentenced him to three consecutive life sentences.

In his charge to the jurors, Dupree said they had three choices -- to find MacDonald innocent or to find him guilty of first-degree murder or second-degree murder. Dupree told the jury that first-degree murder involved a defendant who "coolly deliberates or thinks the matter over" before acting. He said MacDonald should be found guilty of second-degree murder in jurors found that he killed his family but acted without premeditation.

Before sentencing, MacDonald rose with the aid of his attorneys and spoke to the court. "Sir, I'm not guilty," MacDonald told the judge in a clear voice. "I don't think the court heard all the evidence. That's all I have to say."

Immediately after the verdict was announced, St. Mary's Medical Center in Long Beach announced that MacDonald had been fired.

MacDonald and his lawyers had previously protested a ruling by Dupree that the jury could not hear the testimony of six persons who the defense said could link a former drug addict to the crime. At the time of the murders, the MacDonalds lived at Fort Bragg, N.C., where MacDonald served as a Green Beret medical officer. One of his responsibilities was to counsel and treat drug abusers.

MacDonald has maintained since 1970 that his family was slain by four intruders who entered his home while he slept on a living room couch. He testified last Thursday that one of the intruders chanted, "Acid is groovy, kill the pigs" while struggling with him.

Assistant U.S. Attorney James Blackburn argued Tuesday that the club, puncture and stab wounds MacDonald said he suffered in a fracas with the intruders were self-inflicted as part of a cover-up.

Shortly after beginning deliberations, the jury asked to see the pajama top MacDonald said he was wearing when he was stabbed. Jurors also asked for exhibits concerning the pajama top, blood-type charts showing places where blood was found, pictures of Mrs. MacDonald and several other items.

The pajama top was a key piece of evidence for the government. Prosecutors contended that MacDonald, using an icepick, stabbed his wife through the top. They presented an experiment done by the FBI that is said reproduced the pattern of puncture wounds on Mrs. MacDonald's chest by folding the top the way it was found lying over her.

MacDonald said he was attacked while wearing the top and that he later put it over his wife after finding her body in the master bedroom of the house. His attorney, Bernard R. Segal, argued Tuesday that experiments on the top were "sheer fakery."

Segal told the court that his client should be released on bail pending appeals of the verdicts. "There is no interest except vindictiveness" in refusing bail, Segal said.

However, the government argued that MacDonald was a convicted killer, and had reason to flee. "Collette, Kimberly and Kristen are dead," Blackburn said. "This jury said he did it and for that I think he must suffer immediate imprisonment."

Dupree revoked MacDonald's bail and ordered that he be escorted from the court by U.S. marshals.

MacDonald had been cleared of the charges by an Army Board of Inquiry in 1970. However, his father-in-law and mother-in-law, Alfred and Mildred Kassab, pressed the government to bring MacDonald to trial. Partially as a result of the Kassabs' efforts, MacDonald was indicted in January 1975.

After the sentences were pronounced, Kassab told reporters, "This was something that had to be. Now we can rest in peace."