U.S. Postal officials continue to urge people to use the mails as they normally would, but they say that in some cases there will be a day's delay in delivery because of the air controllers strike.

Before the strike, most letters going more than 300 miles generally went by air, but now the trigger distance has been changed to anything over 500 miles. The additional ground transportation needed to deliver the mail is being provided by trucks and trains.

The next-day-delivery guarantee for express mail has been suspended during the strike period. Some express mail is expected to take two days instead.

Joyce Booker, a news information specialist at the Postal Service, reported that in most cases, "There have been no substantial delays of mail delivery at this point. We are still moving the mail, though mailers have been warned to anticipate some delays."

Booker said the Postal Service has set up a control center at Postal Service headquarters as well as at five regional offices to coordinate movement during the strike. She said that the system is successfully moving all types of mail so far and that international airmail has not been affected.

She said postal officials have contingency plans should the number of flights decline dramatically. This would involve moving only certain types of mail by air -- express mail first, then first-class mail and then priority mail.

Several Washington businesses and retailers surveyed yesterday said that they have not experienced any serious delays in their mail service during the air controllers strike.