Continental Airlines and its sister company, Eastern Airlines Inc., yesterday said they will cut ticket prices this week in selected markets to boost sales during the post-holiday season.

The airlines, both units of Texas Air Corp. of Houston, said the fare reductions, coupled with new, more lenient purchase requirements, reverse a trend among airlines to make the industry's most heavily discounted airfares more expensive and restrictive.

In St. Paul, Minn., Northwest Airlines also said it will lower fares and enhance its frequent flier bonuses to boost revenues during the industry's winter doldrums.

"These are positive actions that will stimulate travel during the traditionally slow first quarter," said A.B. Magary, executive vice president of marketing at Northwest.

Other airlines said they were evaluating yesterday's developments. "Suffice it to say that if we compete with them we'll offer the same fares," said Delta Air Lines spokesman Richard Jones.

A spokesman for United Airlines said the airline reduced the advance purchase requirement on its most deeply discounted fare to two days from seven days to match the other airlines.

United will consider fare reductions on a market by market basis, he said.

Miami-based Eastern said it decided to slash airfares for round-trip travel to and from Florida because of weak advanced bookings.

John Nelson, Eastern's senior vice president for marketing, said weak sales apparently are not related to labor uncertainty at Eastern, but instead reflect a general softness in the Florida travel industry.

As an example of the new discounts, Eastern said it will cut one-way fares between some Florida cities and New York, Detroit and Pittsburgh to $79 beginning Wednesday.

Continental said its fare reductions, also effective Wednesday, will save travelers an average of 15 percent to 20 percent.

The reductions will be offered on transcontinental flights, flights to and from Florida and on Continental's midday shuttle between Newark, N.J., and Boston and Washington, among other routes, the airline said.

The most deeply discounted fares on Eastern and Continental will carry certain restrictions, such as a two-day advance purchase, a Saturday lay-over and a maximum stay of 14 days, the airline said.

Some of the fares are effective through Feb. 10, while others will remain in effect longer, the companies said.

Northwest said it has cut its lowest discount fares by as much as $120 for round trips taken through mid-May.

In addition, Northwest chopped the advance purchase requirements for the discount fares from seven days to two.

Examples of Northwest's cuts include a $198 round-trip fare from Detroit to Los Angeles, down from $268, and a $198 fare from Milwaukee to San Diego, down from $248.

Magary said that in addition to reducing its lowest fares, Northwest is taking steps to lower other round-trip and one-way fares.

Northwest is calling its new frequent flier incentive program Mileage Match, an upgraded version of its Worldperks program.

Other major airlines recently announced they will offer frequent fliers triple-mileage credit on trips taken this year.