Wouldn't you know it, the postal rates go up and the darn mail still doesn't get through. But if you were among the intended recipients of more than 800 pieces of mail that didn't make it to addresses in Northwest Washington yesterday, you can't blame the Postal Service. The letters and packages were stolen, right out of the carriers' Jeeps.

On the first business day after the cost of a first-class stamp rose to 18 cents, postal inspectors reported a new outbreak of a problem that has plagued the Postal Service here for months -- the theft of entire bundles of mail left unattended while carriers are making deliveries along their routes. Inspectors said the postal thieves favor affluent neighborhoods, where they look for checks, cash, credit cards and insured packages.

Letter carrier Nicholas Hamilton reported that his vehicle was broken into at about 11:30 a.m. yesterday while it was parked in the 2800 block of 28th Street NW. He had removed some mail for delivery and left the rest of his cargo in the locked Jeep. Six bundles of mail containing about 500 items were stolen, postal officials said.

Then at 12:05 p.m., the vehicle of carrier Jerry Sewell was hit in the same way in the 3500 block of Davenport Street NW. Four bundles of mail containing more than 300 items were reported missing.

Ryland Saxsby, an investigator for the Postal Inspection Service, said the incidents were similar to thefts that took place Feb. 17 and March 2, indicating that the same persons may have been responsible.

Postal inspectors and D.C. police detectives said that the thieves' technique -- familiarizing themselves with postal routes in affluent neighborhoods and breaking into Jeeps when they know the carrier will be out of sight for some time -- resembles that of two men recently imprisoned for possession of stolen mail. They said it was possible those men had associates who are continuing the lucrative operation.

Saxsby said investigators have recovered thousands of pieces of mail stolen in earlier incidents. People to whom the stolen mail is addressed are contacted, by mail, and asked to leave the items with the inspectors while they are checked for fingerprints, he said. Then the mail, found mostly in trash bins and dumps, is sent on its appointed rounds.