“From the hospital end there’s a big feeding frenzy, a lot of bidding going on to bring in doctors,” Mr. Mechanic said. “And physicians are going in so they don’t have to worry — there’s a lot of uncertainty about how health reform is going to play out.”

In addition, Medicare had reduced its set doctors’ fees over the last decade, while insurers have become more aggressive in demanding lower rates from individual practices that have little clout to resist. Dr. Robert Morrow, a family doctor in the Bronx, said he now received $82 from Medicare for an office visit but only about $45 from commercial insurers.

Dr. Cathleen London practiced family medicine for 13 years outside Boston, but recently took a salaried job at a Manhattan hospital. She said she accepted a pay cut because she could see that she was losing ground in her practice. “I think the days of what I did in 1999 are over,” she said. “I don’t think that’s possible anymore.”

The base salaries of physicians who become employees are still related to the income they can generate, ranging from under $200,000 for primary care doctors to $575,000 in cardiology to $663,000 in neurosurgery, according to Becker’s Hospital Review, a trade publication.

Because of the relatively low salaries for primary care doctors, Dr. Suzanne Salamon said that for the last two years she has had trouble filling a prestigious Harvard geriatrics fellowship she runs.

Dr. Howard B. Beckman, a geriatrician at the University of Rochester, who studies physician payment incentives, said reimbursements for primary care doctors must be improved to attract more people into the field. “To get the kinds of doctors we want, the system for determining salaries has to flip faster,” he said.

Doctors can become employees by practicing in a hospital building, or by selling their multispecialty practice to a hospital, so their office becomes part of a network. That has attracted specialists, including many cardiologists who took up such offers several years ago after Medicare reduced physician payments for cardiac procedures like the placement of stents to hold open clogged arteries. The fraction of cardiologists employed by hospitals rose to 35 percent in 2012, up from 11 percent just five years earlier, according to the American College of Cardiology.