By the time you read this, there may be no fluoride in your New York City drinking water.

Don’t panic.

The city’s Department of Environmental Protection shut down the fluoridation system last night for four months to replace equipment at the Kensico Reservoir in Westchester. But officials gave assurances that the interruption would have no significant impact on the city’s dental health.

“It takes a certain amount of time for the benefits of fluoride to accrue to an individual,” said Dr. Mercedes Franklin, director of oral health programs at the city’s Department of Health and Mental Hygiene, “and it takes a certain amount of time for them to diminish.”

Since 1965, New York City has added fluoride to the water supply at a concentration of approximately one part per million.

But the city faces a federal deadline in May to replace the reservoir’s sluice gate operators, which regulate water flow, because they contain mercury, which, though it was, coincidentally, once used in dental fillings, is now considered highly toxic.

To replace the gate operators, the city needs to intermittently shut down the Delaware Aqueduct, which feeds into Kensico, the reservoir just north of White Plains that carries 90 percent of the city’s water, said Charles G. Sturcken, the Department of Environmental Protection’s director of public affairs.

The fluoridation could be switched on and off along with the aqueduct, but Mr. Sturcken said yesterday,

“When you stop and start with the fluoride, the pH gets out of whack.” He said the absence of fluoride would not affect the taste of New York City’s famously tasty tap water.

The debate over whether to add fluoride to New York’s drinking water raged for more than a decade. The Greater New York Committee Opposed to Fluoridation inveighed against involuntary medication by the government. At a 1963 public hearing on the issue that lasted more than 15 hours (“No time had been taken out for meals,” a reporter for The New York Times noted ruefully in a dispatch filed sometime after 1 a.m.), a doctor testified that fluoride caused degenerative lesions in rats. Other skeptics suspected a Communist plot.