A 12 year-old girl who was sexually abused by her father began using drugs, drinking and running away. Finally, she attempted suicide. An 11-year-old who was fondled by a neighbor became withdrawn and cranky and clung to her mother.

Their behavior is not unusual among children who have been sexually molested or abused, experts agree. Many sustain psychic damage that is deep and long lasting.

"Every adolescent I've ever met who's been sexually assaulted has been suicidal in one way or another -- children as young as 9," said Linda Canfield Blick, head of a pilot project in Montgomery County's social services department to treat children who have been sexually abused by a parent or guardian. "They feel lonely, ugly, isolated, guilt-ridden, betrayed and misrepresented."

Blick said many girls fear they have suffered permanent physical damage. Deborah Kelly, a social worker at Prince George's Sexual Assault Center, said boys who are victims of sexual assault often fear they will become homosexuals.

The cases of the two children described about are among a growing number being reported to local authorities. In Prince George's County, a total of 144 suspected cases of child sexual abuse, including incest, was reported in fiscal 1980. Montgomery County reports 76 suspected cases of incest alone for the same period, more than twice the number reported in the previous year. Blick estimates that sexual abuse affects the children in one of every 10 families.

Social workers say the common reactions -- guilt, loss of self-esteem, and fear of being hurt by the perpetrator if they report the offense -- often cause children to remain silent at first.

The average age of a child when he or she ir first assaulted is 8, although the abuse often continues unreported until the victim reaches the teen years, said Kee MacFarlane of the National Center for Child Abuse and Neglect (NCCAN). Then they often tell police or other authorities, such as school counselors. Other cases of abuse are discovered when pregnancy or venereal disease result.

Two young women who have been victims described their experience but asked that their full names not be used.

Marie, a 15-year-old Rockville girl, was sexually abused by her father from the time she was 5 until she was 12. Marie's case was reported to protective services and Blick, the director of the incest project in Montgomery County, introduced the girl's mother Pat to a reporter.

"The way it was presented to me was, it was a game and Mommy wasn't supposed to know about it. I trusted him; he was my dad," she said. Marie says her father bribed her with treats and later threatened to hurt her if she told their secret.

Her mother said the sexual encounters between child and father were extensive. Marie, angry tears welling in her large eyes, said only, "It was a disgusting mess."

Pat discovered the incestuous relationship when Marie was 8. The family sought counseling and the mother said she believed her husband had halted the abuse. The incest never stopped, however, and finally, when Marie was in seventh grade, she told her school counselor.

"I just couldn't stand it any longer . . . It was like I had been carrying a ton of bricks," she said. Because of the emotional chaos "I started doing drugs the same day" and eventually smoked marijuana, took amphetamines and LSD.

The counselor reported the case to the police, who called the mother. Again, Pat confronted her husband in Marie's presence.

"Dad sat there and denied everything," Marie recalled. "I was totally horrified. I was screaming at him, calling him all kinds of obscentites."

The father was arrested and put on probation; the family split up.

Somehow fellow students learned of Marie's troubles and began taunting her at school. She became violent and fought constantly, she said, and when she was depressed or angry she had sexual encounters with "scummy guys" in her school.

By the end of her ninth-grade year, Marie's rampages had landed her in a psychiatric hospital, and later she was placed in the hospital's day-care program. Last September she ran away from the hospital and in December downed 10 tranquilizers and a fifth of Jack Daniels whiskey. Luckily, her stomach did not tolerate the combination. After another six months in a state mental hospital, Marie was released last week.

Marie said she believes she is gaining control of her life again, and is especially relieved that she and her mother are now very close. She has spoken to her father on the telephone, but does not believe he has changed, as he claims. She says she's afraid to see him again.

Unlike Marie, who has a supportive mother, Anne had to suffer through her situation alone. Now 26, the Baltimore woman was severely abused by her father. She was introduced to a reporter by Jack Csicsek, the Maryland coordinator of Parents Anonymous, a support group for families in which physical abuse has occured, to which Anne turned for help.

In addition to abusing her sexually, Anne's father locked her in attics and bashed her in the head with a baseball bat. "There isn't an abuse known to man that I haven't suffered," she said bitterly.

When she was 7, her father began having intercourse with her, and she bore her father's child when she was 13. Anne's father denied his guilt, her mother refused to believe it and neither parent would permit her to have an abortion.

"At first I hated being pregnant then I liked it because I though I'd have something to love me," she recalled. He baby had numerous health problems, however, and lived only four months.

As an adult, Anne has had several miscarriages, the result of a deformed uterus, doctors told her. Anne has also suffered numerous emotional and physical problems, including three nervous breakdowns and experimentation with drugs.

An angry and sometimes violent young woman, Anne has also gone through two marriages. Her first child was taken away by protective service authorities because she beat the youngster.

Anne sought professional help and now says she has learned not to vent her anger at her second child, a 6-year-old daughter who lives with her."She gets all the love and attention and affection every child should get," Anne said. Most importantly, she said she has taught the child to know how and where people should not touch her.

Preventive techniques such as Anne has taught her daughter are important. Wanda Thompson of the Bethesda Crisis Center's sexual assault service believes parents must be more alert to signals from their children. Warning signs that a child may be a victim of abuse include loss of appetite, irritability, changes in school behavior or regressive behavior such as bedwetting or fantasizing. Adolescents often become rebellious.

Most child abusers are fathers, stepfathers, relatives and friends of the family, said Linda Blick. Deborah Kelly pointed out that the abuser can be an older child in the family or in the neighborhood, and advised parents to investigate significant changes in relationships between children, particularly if the youngsters are far apart in age.

In both Montgomery and Prince George's counties, police and protective service workers investigate all suspected cases of sex abuse, and try to interview the child victims alone as soon as possible. They often give young children "anatomically correct" rag dolls to play with during interviews so the youngster can show what happened instead of trying to put it into words, a technique they say is frequently successful.

Jean Albright, a social worker at the Montgomery project for helping families in which incest has occurred, said youngsters often are blamed by other members of the family who may accuse them of seducing the offender or of putting themselves in vulnerable positions, for example, by hitchhiking or staying out late.

Albright added that denial of the abuse by someone the child trusts can be davastating to the youngster. Often in incest cases, the offending relative is ordered to leave the home, or if the family dynamics are particularly bad for the child, protective services will place the child under temporary foster care.

National statistics suggest that 70 percent of runaway teen-agers have been sexually abused by someone close to them. Albright said many prostitutes, drug addicts, battered spouses and sex offenders were sexual abuse victims as children.

Another phenomenon being reported by the national center for child abuse is that 25 percent of assaulted children are revictimized by different offenders. The center's MacFarlane believes this is because such children show their vulnerability in their quest for attention and affection and respond to sexual advances because they have been conditioned to them from a young age.

Montgomery and Prince George's counties offer group and individual counseling to victims of sex abuse. Prince George's Kelly said persuading children to talk about their experiences is crucial, because they need to vent their anger and to know that others have had the same problems.

Albright said it is vital that child victims receive treatment and are helped to regain their self esteem. She and other Montgomery social workers like to bring mothers and daughters together after both have had counseling. Often the two must establish communications that never existed before, she said. Whenever possible, Albright said, offending fathers should "have a chance to apologize to their daughters and take responsibility (for their actions)."

CORRECTION: In the first part of this story, which appeared last week, Prince George's County police detective James W. Warner was incorrectly identified.