MANUFACTURERS HAVE OFFICE-FASHION ANSWER.

The biggest maker of pants and the biggest maker of shirts are shaking up their wardrobes.

Levi Strauss & Co. is bringing out a line of men's dress trousers intended to fill the wardrobe gap between khakis and designer dress slacks or suit pants - and in the process become the third major Levi Strauss brand in the company's 140-year history, after Levi's jeans and Dockers casual slacks.

The new line, named Slates, is being peddled in an elaborate $20 million advertising and promotional campaign.

Meanwhile, Arrow Co. is testing in five markets across the country a humorous television spot that pitches casual shirts for men rather than Arrow's traditional dress styles.

Those moves are indicative of how apparel makers are scrambling to meet changing consumer needs, particularly as men continue to demand casual fashions.

``The `casualization' of America has really changed the way men dress,'' said Nancy Friedman, marketing director for the Slates brand at Levi Strauss in San Francisco. ``As suits decline, men say they need more options, more alternatives, because they don't know what to wear.''

``There's confusion,'' agreed Phil Molinari, president of the Arrow division of Cluett Peabody in New York, ``because all these businesses have said you can dress casual. But what do you do in a culture like IBM's, where you were told to wear navy blue suits for 100 years? They don't have anything in their closets.''

Levi Strauss has tried selling dressier slacks under several brand names including Dress Dockers which, Friedman said, is being discontinued to focus on Slates.

Annual sales of men's casual pants, according to the NPD Group, a market research company, are $2.8 billion, compared with $1.9 billion for men's dress pants.