Automated teller machines, those money-spitting, 24-hour cash kiosks, are the hottest banking convenience since the drive-up window.

But how does the cash get in the machines? Why don't they ever seem to run out of money? It's a touchy subject, one that most banks just don't talk about.

"We prefer not to profile how we replenish our ATMs," said Jerri Franz, a spokeswoman for Jacksonville, Fla.-based First Union National Bank of Florida. "It's dangerous enough for the people out there."

It's difficult to find out who "the people out there" are. In most cases they don't work for the banks but for private contractors, such as Wells Fargo.

With more than 75,000 machines in service nationally there is a tremendous need for cash replenishers. A busy machine will handle more than 5,000 transactions a month.

At the banks, cash for the machines is sorted by a "discriminator," a machine that scans bills and sorts them by quality. Most ATMs can handle only the highest quality bills.

ATM replenishment teams pick up cash at secret vaults and flit like bees from machine to machine, responding to computer distress signals that tell the home office when the cash supply is dwindling. Replenishers are also called on to fetch customer deposits and to retrieve money inadvertently left behind by customers.

Replenishment teams place a premium on secrecy. They typically travel in unmarked armored vans on routes that vary daily.

Cash handlers are usually armed. Some wear insignias, while others wear jeans and carry their cargo in gym bags. All are told to avoid conversation with passersby and to never admit they are carrying cash. If pressed on the matter, they are instructed to say that the machine has malfunctioned.

Bankers would not say how much cash a typical ATM carries or how often each is refilled. But Ed Kalail, a spokesman for ATM-maker Diebold Inc. in Canton, Ohio, said the machines can store as many as four "cassettes," each packed with 3,000 bills. Assuming an equal mix of tens and twenties, a fully stocked ATM could carry up to $180,000. For security reasons, however, Kalail said few banks stock spare cassettes.

Replenishers remove empty cassettes and replace them with full ones, punch into the machine's computer the amount of the refill, flick a switch, and it's cash-to-go time. Usually, the replenisher never touches cash.

ATM security is serious business. Each machine is equipped with an alarm system, a telephone and a computer link to a central monitoring center. Cash cassettes are locked in a steel or concrete safe.

Replenishers typically are required to check in with the home office before they leave their armored vans. If they don't report back within a few minutes, emergency measures are taken.