Claus von Bu low's former lover testified today that von Bu low told her he stood by in December 1979 and watched his wife reach "the point of dying" after ingesting a combination of alcohol and barbiturates. But, she said, he found he "couldn't go through with it," called a doctor and saved her life.

Alexandra Isles, 39, in a dramatic day of testimony at von Bu low's retrial on charges that he twice tried to kill his wife, heiress Martha (Sunny) von Bu low, with injections of insulin, told the jury von Bu low said he knew his wife "was in a bad way."

"He said they had been having a long argument, talk, about divorce that had gone late into the night. She had drunk a good deal of eggnog. He said, 'I saw her take the Seconal,' " Isles told the jury.

"He said he watched her all day, knowing that she was in a bad way, all day, and watched her and watched her and finally when she was on the point of dying he said he couldn't go through with it and called and saved her life."

As Isles recounted the conversation, which she never before had revealed in court, von Bu low shook his head once and looked up at the ceiling. His face was flushed. The prosecution contends von Bu low wanted to kill his wife in order to inherit his share of her fortune and marry Isles, a former soap opera actress.

Throughout her testimony, von Bu low stared intently at a gold pen he held in both hands and wrote occasionally on a small black note pad. Isles and von Bu low exchanged a look at the outset of the day's proceedings, but for the most part they awkwardly but deliberately avoided each other's eyes.

During her testimony, Isles described how her relationship with von Bu low had continued after the conversation about the December 1979 incident. Under cross-examination by defense attorney Thomas Puccio, Isles repeated that von Bu low had told her he had saved his wife's life by calling a doctor.

"You certainly didn't believe he intended to harm his wife or you wouldn't have associated with him. Isn't that true?" Puccio asked Isles.

"I'm ashamed to say it's not true," Isles testified. She sighed.

At a hearing earlier, with the jury outside the courtroom, Isles, in a blue blouse and wearing blue combs in her thick dark hair, testified that she had not told prosecutors about her conversation with von Bu low until late yesterday after her arrival from Frankfurt.

Isles flew in at the eleventh hour to testify after Judge Corinne P. Grande told prosecutors they could not use transcripts of her testimony at the first trial in 1982 and would have to produce her in person if they wanted the jury to hear what she had to say.

Isles, whose testimony today was punctuated frequently with sighs and long pauses, said that when she appeared at von Bu low's first trial, "my purpose . . . was to answer in the most minimal way what I was asked because I had -- I guess because I couldn't cope . . . I couldn't bear to think about it, so I withheld it."

At that first trial, Isles testified only that von Bu low had told her that Sunny von Bu low's 1979 coma was brought on by a combination of pills and liquor.

During his cross-examination of Isles today, Puccio also quizzed her about a letter she wrote to von Bu low after that first trial ended in his conviction for attempted murder.

Puccio, rapidly firing questions, asked Isles if she had said in the letter that she still loved von Bu low.

"When I wrote that letter I didn't know what I was doing," Isles said.

When Puccio persisted, she said, "What if I changed my mind the next day and regretted it?"

Again Puccio asked her if she had said in the letter she was jealous of people who were spending time with von Bu low.

"Perhaps," Isles responded.

"As a matter of fact, he didn't want to continue the relationship," Puccio stated to the witness. "I don't blame him," responded Isles, whose testimony at that first trial was cited by jurors as crucial in their decision to convict von Bu low.

"I can't stand by that letter . . . I was terribly upset when I wrote it," Isles said, her voice rising to a combative tone.

And suddenly Isles turned the questioning on Puccio.

"Have you ever been in love?" she asked the defense lawyer, adding quickly, "I doubt it."

"Maybe I can tell my wife to answer that . . ." Puccio shot back as he ended his questioning.

Observers of the first von Bu low trial had said Isles had been nervous and stiff. Today she was at ease. The first time she also seemed glamorous and high society. But today her look was more that of a young Irish colleen. Her hair pulled away from her face, Isles wore no makeup and wore a simple skirt and blouse. But whenever she left the witness stand, one of her lawyers always took her by the arm and led her demurely through the corridors, trailed by reporters.

Prosecuting attorney Marc DeSisto began his questioning of Isles in a soft, gentle voice.

Isles started by telling the jury she met von Bu low at a luncheon for a mutual friend in 1978 and in the first two to three months of 1979, "well, it became a loving relationship."

"Did it become intimate?" DeSisto asked almost reverently.

Isles nodded.

"When did the relationship become intimate?" the prosecutor asked.

"In March," Isles said, blinking once. Von Bu low sat at the defense table, rubbing his chin.

By April they talked of marriage, "I think in my kitchen," Isles told DeSisto. In that same month, Isles testified, she set a deadline for von Bu low in terms of the progress of their relationship -- a deadline the prosecution characterizes as an ultimatum, which it claims led von Bu low to attempt to kill his wife.

"Yes, I said that I thought that I (sigh) that it was my wish and hope that we could be together as a couple by Christmas of that year and would that be enough time for him to get his affairs together . . . he said that it would," Isles testified.

The couple talked about finances. "It would be important for Claus to get a job so he could support a family," Isles testified.

Isles told the jury that by midsummer, however, she was seeing less of von Bu low.

"I think it was (sigh) I think it was the beginning of problems," Isles said.

That July, the couple went to London, but she said she did not see him often after that. "Well, there was a lot of tension between us and just general tension and it wasn't as happy," she said and then sighed into the microphone at the witness stand.

"I think that most of it had to do with him von Bu low having a very hard time finding work."

In December 1979 Isles traveled to Ireland, but before she left, she said, von Bu low talked with her. "He said he would be a free man after Christmas . . . he would have come to an understanding about divorce with his wife."

But on Dec. 27, von Bu low called Isles again, she testified.

"He said that he would not be able to meet us Isles and her young son at the airport . . . because his wife had been taken to the hospital in a coma." Sunny von Bu low recovered from that coma.

In early January 1980, Isles said, von Bu low called her and related the additional details about the eggnog and the barbiturates and about his decision to save his wife's life.

Isles testifed that the following May, however, von Bu low told her that it "was in fact hypoglycemia that had caused the coma and it had nothing to do with anything she had taken herself."

The couple began seeing each other again, but by September, their relationship was once again waning, Isles testified. Asked why this had happened, she sighed again and said, "Well, I guess I wasn't trusting him so much."

Isles said von Bu low had told her he was moving out of his family apartment at 960 Fifth Ave., but she indicated during her testimony that she was not convinced.

In early December 1980 she received a letter from von Bu low that his wife had suffered a concussion and was hospitalized. That month she saw him twice, returned some gifts he had given her, which she dropped off at the Fifth Avenue apartment, and later delivered some Christmas presents to him there.

Then, on Dec. 21, she received a call from von Bu low, Isles said. "He said that his wife had another coma and that she was very ill and had gone to the hospital."

Sunny von Bu low has not recovered from that coma, which doctors say is irreversible.

The following month, January 1981, the couple still had a relationship, she said.

"I think it was loving . . . loving," Isles testified, emphasizing the final word in a loud voice. That February, they went to the Bahamas and talked.

In March, the two traveled to Florida with von Bu low's daughter, Cosima, and Isles' son. From then until the following December, Isles said, the couple saw each other eight to 10 times.

Isles also testifed under questioning by the prosecutor that in January 1982 she asked von Bu low about a report concerning Valium paste that she had read in connection with the trial.

There has been prosecution testimony from von Bu low's stepson, who said he found that substance in a black bag found among von Bu low's belongings. The black bag and its contents are key elements in the prosecution's case.

Isles was asked whether von Bu low had told her what was done with the Valium paste.

"He said he had made some up for his wife that she could use in her nose because that way Maria Schrallhammer could not know what she had in her medicine." Schrallhammer, who has appeared as a prosecution witness, was Sunny von Bu low's personal maid.

During cross-examination, Puccio repeatedly asked Isles if she was suspicious that von Bu low did not want to marry her, that he wanted her to continue to be his mistress. Puccio asked her if it was correct that she wanted to marry him.

"Yes. I loved him," she testified. And no, she said, she didn't think that all von Bu low wanted was a mistress.

"No, he was very generous but I really felt he wanted to make a new start of his life," Isles said.

Isles denied Puccio's assertions that she had repeatedly asked von Bu low to marry her and that she had pressed von Bu low to tell his wife about their relationship.

Isles said that in December 1979, she was hopeful von Bu low would tell his wife of her existence. "Well, I hoped, I hoped," she said. "At that time he was hysterical and in very, very bad shape," she said.

"I felt he didn't have the strength to pull his own life together. We loved each other. I believed that."

In 1980, Isles told Puccio, "I still loved him." And under further questioning she added, after sighing yet again, "1980 was a very confusing year. I wasn't sure from then on whether I did want to marry him or not. I loved him."

"No matter what the cost, you wanted Mrs. von Bu low to know you existed," Puccio asked her again.

"I don't remember having such an ill feeling toward her," Isles said.

Puccio reminded Isles of her testimony that von Bu low had once telephoned her claiming to be in Geneva when in fact he was in New York. He then asked Isles if she then became concerned that von Bu low was trying to "dump you, so to speak."

"I became concerned that he was crazy," Isles said, referring to von Bu low's saying he was abroad.

When Puccio again pressed Isles on whether she wanted Sunny von Bu low to know about her, Isles shot back, "You keep asking me that and I'd like to know why you keep asking me that, because you may know something I don't know."

Isles also denied she had intended for Sunny von Bu low to discover the items she had returned to Claus von Bu low and which she personally delivered to their Fifth Avenue apartment. When Puccio asked her if she didn't think Sunny von Bu low would see those items, Isles said, "She was not a housewife who would pick up things at the front door."

Isles acknowledged that von Bu low told her later in a letter that on the same day she returned these items, Sunny von Bu low was hospitalized in New York after taking what Puccio described as an "incredible quantity of aspirin."

Again Puccio asked her about wanting Sunny von Bu low to know about her.

"You keep using the words 'persist' and 'push' and it wasn't like that," Isles said.

"Well, you wanted to be with Mr. von Bu low," Puccio continued.

Said Isles, "I loved him."