A technological revolution took place inside Metrobus No. 9489 yesterday morning, but it was so subtle that many riders didn't notice.

The bus is one of 80 Metrobuses outfitted with new electronic fareboxes that accept the SmarTrip card, the rechargeable plastic card used by subway riders since 1999.

The SmarTrip card, which is "loaded" with a prepayment and waved over a reader that deducts the fare in seconds, signals an end to a major hassle faced by bus riders: the need for exact change.

"People are always digging for their money, especially on a day like this," said Metrobus operator Patricia Partee, who was driving bus No. 9489 through the rain yesterday on the 25J route between Ballston and the Pentagon. "I think the more people learn about the new fareboxes, the more they'll start using the SmarTrip card."

While the new fareboxes still accept cash and tokens -- but no pennies -- they give riders the option of using the same SmarTrip card they wave over subway turnstiles.

The fareboxes are being tested on 80 buses that circulate in Northern Virginia, or about 5 percent of Metrobuses.

If the 90-day test goes well, the fareboxes will be installed on the rest of the Metrobus fleet by spring. The project is costing Metro $24 million.

The debut of SmarTrip cards on Metrobus is the first step toward a seamless regional transit system.

Metro is poised to sign an agreement with 16 other bus and rail systems in the region to install fareboxes that will accept the SmarTrip card, so that a Metrobus rider could use a SmarTrip card to pay for a trip on Virginia Railway Express as well as the Montgomery County Ride On or the Baltimore bus system, for example.

Many riders who boarded Partee's bus yesterday morning flashed their seven- or 28-day bus passes or paper transfers and didn't give the farebox a second glance.

For those riders, who pay a flat amount for unlimited rides during a specific period, SmarTrip cards remained a foreign object.

"Doesn't matter to me," said Walter Macon, who rides three Metrobus routes each day from the District to reach his job in Arlington, and buys seven-day passes for unlimited rides because it's the cheapest way to travel.

While subway riders have embraced the SmarTrip card -- 280,000 have been sold in three years with little marketing -- it is unclear whether bus riders will follow suit.

The card costs $5 and requires the bearer to prepay any amount up to $200. Several Metro board members have questioned whether that poses an obstacle to bus riders, who are generally less affluent than subway riders.

Some passengers aboard Partee's bus were bothered by the fact that new fareboxes don't accept pennies. In fact, a nerve-jangling alarm rings three times if someone tries to insert the copper coins and the machine unceremoniously spits them out.

"I always just dump stuff in there and go," said Claudine Blanc, 30, one of several people who mistakenly inserted pennies into the machine and set off the alarm. "I'd rather have the old fareboxes back."

Metro may change the new fareboxes to accept pennies if enough complaints are received during the 90-day test period, officials said.

At least one rider aboard Partee's bus was excited to see the new fareboxes. "I've always been upset that you couldn't use the SmarTrip card on the bus," said Kurt Eddy, a 25-year-old Falls Church resident. A man of his generation, Eddy carries no cash and prefers electronic transactions for all purchases. "Everywhere you go, everyone takes American Express," he said. "We have SmarTags on the highway, we should have SmarTrip on the buses. It's just more convenient."