Metro was up to its institutional armpits yesterday in quarters, the result of its decision to charge 50-cent cash fares on both its subways and its buses on the Fourth of July. "And our people are still flattening out dollar bills," said Metro's public affairs director, Beverly Silverberg.

An estimated 250,000 trips were made on the subway system, she said, and most of them were paid for in quarters and dollar bills. "There were very few nickels and dimes," she said.

As a Metro rider, I used six quarters to get to and from the office on Monday. It got me to thinking: The quarter apparently has become the primary coin of the realm, needed for coin laundries, school lunches, vending machines, video arcades, parking meters, transit fares and other purposes for which a nickel or dime once sufficed.

Figures from the Bureau of the Mint, while not conclusive, reinforce this impression. Between 1961 and 1982, the net annual demand for quarters distributed through the Federal Reserve banks has increased from 118 million to about 1 billion, about twice the rate for dimes, which increased from 280 million to about 1.1 billion. That doesn't count coins in circulation that are not redistributed through the Federal Reserve, but it's a measure of their use.

The lowly penny, which many of us will no longer stoop to pick up, has gone from 2.2 million to 15 billion. The apparent reason: sales taxes.