Still, since the beginning of 2005, Lilly has cut its United States work force by more than 8 percent, reducing it to 22,000 jobs by last January.

Lilly also noted that its overall reported worldwide tax rate for 2006 — which includes taxes paid to other countries and taxes that it has deferred but will theoretically pay at some future date — was about 20 percent in 2006.

Pfizer, Merck and Amgen declined requests for comment.

Tax experts like Michael J. McIntyre, a law professor at Wayne State University in Detroit, say the drug makers are taking advantage of antiquated rules that work better for manufactured products like steel and automobiles.

Under this system, when companies transfer products between divisions in different countries, they must account for the sales internally through “transfer pricing.” But they have significant discretion in how they set prices for these transactions.

That turns out to be especially so for high-margin products like drugs, which in pill form cost only a few cents each to make once they have been invented, but can be sold for several dollars apiece. The hefty profit margins result in part from patents that can protect the drugs from competition for years. And by transferring those valuable patents overseas, companies can declare that their profits should follow the patents overseas as well.

Under the rules of transfer pricing, if a company moves patents or other so-called intangibles from its United States division to a foreign subsidiary, the foreign unit is supposed to pay the American division a fair-market price. But outsiders have a difficult time determining if companies have properly assessed the value of patents, trademarks and other intangible properties.

To further complicate matters, some corporate subsidiaries in tax-haven countries, like Singapore and the Netherlands, now directly finance research in the United States. So they own the patents without ever having to “buy” them from their American parents, Mr. McIntyre said.