WASHINGTON -- The makers of the impotency drug Viagra and the painkiller OxyContin said Monday they will add radio transmitters to bottles of their pills to fight counterfeiting.

The technology will allow the medicines to be tracked electronically from production plant to pharmacy, a development the Food and Drug Administration said is an important tool to combat the small but growing problem of drug counterfeiting.

The devices will be part of the large bottles that manufacturers ship to drug stores and wholesalers, not the containers that consumers take home from their pharmacies. Most counterfeiting occurs in the wholesale distribution of medicines, FDA officials said.

Shipments of OxyContin bottles with the transmitters will begin this week to two large customers, Wal-Mart and wholesaler H.D. Smith, the drug manufacturer, Purdue Pharma, announced.

Pfizer Inc. plans to start shipping bottles of Viagra with radio frequency identification, or RFID, by the end of next year, Pfizer spokesman Bryant Haskins said.

"We're starting with Viagra because it is probably the best-known and one of the most counterfeited pharmaceutical products," Haskins said.

Pfizer, which sells more than $1.5 billion worth of Viagra a year, said it will spend several million dollars to add RFID tags to Viagra bottles.

OxyContin is a powerful narcotic that has become a target for drug abusers who figured out how to use it for a quick, heroin-like high.The new bottles also should help authorities and the company in its battle against theft of OxyContin from pharmacies, Purdue Pharma security chief Aaron Graham said.

"If a police officer catches someone with a couple of bottles, we can trace them back to the pharmacy they were stolen from. That's a huge step forward," Graham said. The company said it will donate 100 hand-held scanners that can read the labels to law enforcement agencies.

Purdue Pharma also will be taking other anti-counterfeiting measures for OxyContin, including the use of color-shifting inks.

A third pharmaceutical company, GlaxoSmithKline, said it too will begin using RFID on one of its products in the next 12 to 18 months.

An FDA report earlier this year concluded that radio transmitters should lead the way in fighting drug counterfeiting. But the Bush administration declined to order pharmaceutical companies to adopt the technology or other measures to combat the problem.

Still, administration officials said they expect widespread use of RFID by 2007.

The FDA, which must approve labeling changes, said Monday it would allow drug makers that use RFID wide latitude in designing the new labels.

In the late 1990s, the FDA conducted an average of five investigations of counterfeit drugs per year. Since 2000, that figure has risen to more than 20 investigations per year. Last year, federal officials stalked counterfeit versions of Procrit, which helps people with cancer and AIDS combat anemia, and Lipitor, a cholesterol-busting drug. The fake Lipitor prompted the recall of more than 150,000 bottles in 2003.

The RFID tags look like ordinary labels but are really computer chips with antennas wrapped around them. The tag works like a passport, picking up a notation at each stage of the distribution chain when the chip is activated. Sensors at distribution centers use radio waves to activate the tags, which are electronically read and stamped with a record of where they have been.

A counterfeit drug would have no such record.

Federal officials worked through the kinks in a $3 million pilot project that included pharmaceutical manufacturers Johnson & Johnson, Merck & Co. and Wyeth and such retailers as CVS Corp. and Rite Aid Corp.