When Jane Fonda's older child, Vanessa Vadim, went off to college in 1985, Fonda suffered a mother's normal anxieties. "I often find myself imagining worst-case scenarios," she wrote in the Ladies' Home Journal. "She's going to run away, become a drug addict..."

As it turned out, Vanessa, now 21 and a Brown University senior, apparently spent an uneventful three years studying in Providence, R.I., with occasional forays into social activism. She took part in a pro-choice march on Washington, D.C., last spring and helped build a school in Nicaragua. But this month Fonda got one of those phone calls a mother dreads most: Her daughter by French director Roger (Barbarella) Vadim had been arrested at 8 A.M. near a seedy Manhattan tenement where drugs are sold. Though there is no evidence that Vadim uses drugs, police say the man she was with, Thomas Feegel, 22, was carrying two glassine envelopes of heroin.

According to an undercover narcotics officer, Vadim was arrested after she stepped next to Feegel and shouted, "If you're going to arrest him, you have to arrest me!" The policeman obliged. It was then that Vadim reportedly said, "We were only buying the drugs for a school project." The authorities found that explanation unpersuasive, since Feegel allegedly had a hypodermic needle. Vadim, who never mentioned her famous parents, spent a night in jail before pleading not guilty and being released without bail. She was not arrested on drug possession charges but faces up to six months in jail and a $1,000 fine for obstructing an arrest, loitering and disorderly conduct. Feegel, also released without bail, faces a possible year in prison on a felony drug charge.

It was a sobering morning-after following a glittering night-before. Vadim had come to New York City to be with her mother at the splashy premiere of her latest movie, Old Gringo. But later, when Fonda repaired to the elegant Regency Hotel, Vadim turned up downtown with Feegel, a graduate student in a New York University writing program. Vadim and Feegel, who is the son of a former Atlanta medical examiner, are "just friends," according to Va-dim's attorney, Robert Kalina.

Vadim's parents quickly spoke out in her support. Fonda reportedly showed no anger when her daughter rejoined her at the Regency. Said Fonda: "Vanessa is a good daughter and a serious student of whom I am very proud, and I stand behind her through this."

Once released, Vanessa telephoned her stepfather, Tom Hayden, and her father. "She said she was really annoyed with herself," says Roger Vadim, 61. "She said spending a night in jail was nothing compared to her misgivings about the worry she probably caused us." But Vadim is confident Vanessa has done nothing wrong. "Any problems Vanessa has are connected with her mother's image," he says. "Any child would have trouble growing up in the shadow of such a strong image. At college, she joined the drama association but had to give it up. She said, 'I'll never be as good as Mom.' Now she's interested in a career in journalism. She wants to make her own mark."

—James S. Kunen, Victoria Balfour in New York and Georgina Oliver in Paris