The contrast between 18-year-old actress Melissa Sue Anderson and Mary Ingalls Kendall, the blind girl she plays in ''Little House on the Prairie,'' is perhaps the most dramatic in Hollywood.
In the syrupy weekly series Mary is a wimp. She is victimized by man and God. She is dowdy. Her clothes are drab. She wears no makeup. Her life is burdonsome.
Off-screen, Melissa is a glamour girl. She has dated Frank Siatra Jr., 37, for two years. She lives in her own San Fernando Valley home and drives a fancy car. Her beautiful, pouty face is impeccably made up.
Her pale blue eyes -- not unlike a Siamese cat's in color and shape - are lively, flirty and knowing beyond her years. She is anything but a wimp.
Melissa's clothes are chic and fashionable. One day recently she wore an orchid jump suit made of parachute silk with zippers at ankles, wrists, neck and elsewhere. Her figure is as faultless as her young and pretty face.
Her cheeks were pink with indignation at a recent story in a national newspaper which reported she was leaving ''Little House on the Prairie'' because of a long-simmering beef with her co-star, 17-year-old Melissa Gilbert.
It's true, Melissa Sue will be appearing in only three episodes of ''Little House'' this season. It is also true another regular actress in the series, Alison Arngram, will not appear in any.
''The story is absolutely false that there has been any big blowup between me and Melissa and Alison,'' Melissa Sue said heatedly.
''Alison is leaving because of a contract disagreement with the producers. I'm leaving because there just isn't much more that can be done with the blind character I play.
''The writers have exhausted all the possibilities for Mary Ingalls Kendall. First of all, she became blind. Then she had a miscarriage. She lost her mind. Another baby died in a fire. She's had all kinds of diseases.
''It is just a matter of moving on for me. I've done about everything I could do with the character.
''There have been rumors for years that Melissa Gilbert and I didn't like one another. They said I was jealous of her, that she was jealous of me. Then this story came out saying Alison and I were leaving because we resented Melissa.
''I've never been jealous of Melissa. And she certainly hasn't been jealous of me.
''When I read the story I telephoned Melissa and we got together to talk about it. We laughed, but it really isn't funny. It hurt us both.''
Melissa Sue is grateful to ''Little House on the Prairie'' and aware that the show, now going into its eighth year, is responsible for making her a star of sorts.
Melissa starred in ''Happy Birthday To Me,'' a horror movie released earlier this year. Her performance portends a happy future in feature films.
As Melissa says, she has no real competition for her teenage movie roles, noting that Brooke Shields and Kristie McNichol are both very different types.
She is leaving ''Little House'' with few regrets. She was 11 years old when the show began. Melissa virtually grew up in the role of the frontier child.
Since turning 18, Melissa has not had to be accompanied on the set by a social worker. Her life with Sinatra Jr. is sophisticated. She sees the singer four or five times a week.
She is delighted with her independence, free of supervision. Melissa has enormous self-confidence and a quiet detachment from her associations on ''Little House.''
''I'll miss the people,'' she said. ''They're all very nice. Not family, but friends. I'm sure I'll be seeing them on other projects in the future.
''Actually, I haven't been in as many episodes of the series in the past few years as I was at the beginning. I started doing fewer shows when Mary went blind. The series has been a good experience for me personally and professionally.
''It's made me financially independent and will keep me that way in the future. The show goes into syndication this year and then there are the foreign sales. I'm paid residuals on all those things.
''It's good that I've established myself at 18 as an actress that people recognize. They'll pay to see me. But I wouldn't recommend getting into the movie and TV business at my age.
''Unless you look like Brooke or you are discovered like Cathy Moriarty or can act like Jane Fonda, you can't get work in this business. There are too many actresses and too few jobs.
''I'm lucky people know my work and that I have loyal fans. Directors and producers have seen me do other things besides 'Litle House.' Every year during hiatus I made sure I did at least one TV movie or guest shot.
''Now I'm going to concentrate on feature movies and TV films. Another series wouldn't interest me unless it was really outstanding.''