At the Washington offices of Burson-Marsteller, which handles public relations and polling for a variety of corporate and political clients, so-called associates typically make $40,000 to $50,000 a year, and often work well beyond 40 hours a week. Some are tasked with pitching in on 24-hour-a-day monitoring of media coverage for clients in addition to their usual work, which can keep them up late into the night.

Under the previous federal overtime rule, which applied automatically to most salaried employees making less than $23,660, those additional hours were essentially uncounted, making the young associates a relative bargain. (Employees making more than this can be eligible for overtime under a much more subjective “duties” test, but often don’t receive it.) Under the new rule, many of these staff members are to be paid time-and-a-half overtime when they work more than 40 hours a week, if their salary remains unchanged.

Catherine Sullivan, a company spokeswoman, said: “Burson-Marsteller has always paid, and will continue to pay, overtime to those who are eligible.” She noted that employees below the associate level currently do receive overtime pay and also participate in round-the-clock monitoring.

Some organizations in which young staff members are already relatively well compensated said they would probably raise salaries over the new threshold.

But the economics of that approach may be less practical at smaller companies where labor represents a larger share of overall costs. A former employee of the Wylie Agency said assistant literary agents there — usually eight to 10 in the New York office — typically earned in the $30,000s and routinely worked 50 to 60 hours a week without overtime pay.

The former Wylie employee, who spoke on the condition of anonymity because of fear of reprisals, said that there was such an expectation of long hours that anyone arriving after 9 a.m. or leaving before 6:30 p.m. generally felt compelled to email the entire office, giving a reason for being AWOL.