Nicotine is highly addictive, and while no one has studied the effect of the increases on smokers, the higher levels theoretically could make new smokers more easily addicted and make it harder for established smokers to quit.

The trend was discovered by the Massachusetts Department of Public Health, which requires that tobacco companies measure the nicotine content of cigarettes each year and report the results.

As measured using a method that mimics actual smoking, the nicotine delivered per cigarette -- the "yield" -- rose 9.9 percent from 1998 to 2004 -- from 1.72 milligrams to 1.89. The total nicotine content increased an average of 16.6 percent in that period, and the amount of nicotine per gram of tobacco increased 11.3 percent.

The study, reported by the Boston Globe, found that 92 of 116 brands tested had higher nicotine yields in 2004 than in 1998, and 52 had increases of more than 10 percent.

Boxes of Doral lights, a low-tar brand made by R.J. Reynolds Tobacco Co., had the biggest increase in yield, 36 percent. Some of this may have been the result of an increase in the total amount of tobacco put in that brand's cigarettes, one expert said.

The nicotine in Marlboro products, preferred by two-thirds of high school smokers, increased 12 percent. Kool lights increased 30 percent. Two-thirds of African American smokers use menthol brands.

Not only did most brands have more nicotine in 2004, the number of brands with very high nicotine yields also rose.

In 1998, Newport 100s and unfiltered Camels were tied for highest nicotine yield at 2.9 milligrams. In 2004, Newport had risen to 3.2 milligrams, and five brands measured 3 milligrams or higher.

"The reports are stunning," said Matthew L. Myers, president of the Campaign for Tobacco-Free Kids. "What's critical is the consistency of the increase, which leads to the conclusion that it has to have been conscious and deliberate."

"People need to be aware of this," said Sally Fogerty, Massachusetts's associate commissioner for community health. "If a person is trying to quit and is having a hard time, it's not just them. There is an increasing percentage of nicotine that they are ingesting, and that may make it more difficult."

The Centers for Disease Control and Prevention also focused on the potential behavioral consequences of the finding.

"We know nicotine is addictive, so if the amount of nicotine in cigarettes is increasing, it could make it even harder for the 70 percent of smokers who want to quit and the more than 40 percent who try to quit every year," Corinne Husten, acting director of the CDC's Office on Smoking and Health, said in an e-mail message.

No spokesman for a tobacco company would speak on the record about the Massachusetts findings yesterday.

One company official, speaking on the condition of anonymity, said that while the nicotine content measured by smoking machines can vary by up to 6 percent between individual cigarettes of the same brand, "we don't know" whether an entire brand's production could differ that much from year to year.

But in a 1,653-page opinion released two weeks ago in a landmark suit against the major tobacco companies by the federal government and several anti-smoking organizations, the judge found that cigarette makers adjusted nicotine levels with great care.

"Using the knowledge produced by that research, defendants have designed their cigarettes to precisely control nicotine delivery levels and provide doses of nicotine sufficient to create and sustain addiction," wrote U.S. District Judge Gladys Kessler.

The ruling enjoined the companies from misinforming the public about tobacco's hazards. The companies are uncertain what that means and cited the ruling yesterday as the chief reason for their silence. Reynolds and Lorillard Tobacco Co. have also temporarily shut down their Web sites.

Reginald V. Fant, a clinical pharmacologist and nicotine expert at Pinney Associates, a consulting firm in Bethesda, said increasing nicotine content by 10 percent "would not be expected" to change how much a person smokes but could affect his ability to quit.

"We know that physiologically the changes in the nicotine receptors in the brain are related to the amount of nicotine consumed," he said.

Neal Benowitz, a physician and pharmacologist at the University of California at San Francisco, said, "I don't think we know what the consequences are for the population in terms of addictive behavior and how hard it is for people to quit."

Myers said the Massachusetts findings are evidence that tobacco products should be more strictly regulated.

"The only way the companies were able to secretly increase nicotine levels without anyone knowing about it is because no federal agency regulates tobacco products," he said.