A 22-year-old man walked up to a Money Exchange machine in Crystal City around 11 p.m. one night last spring and began a transaction. Moments later, another man stepped up behind him, pulled out a small dark revolver and demanded the $50 that had just spewed forth from the machine. The robber then fled with the cash--and the keys to the victim's Pontiac Sunbird.

"He wanted $1,000," an Arlington Police Department official said, "But of course the machine doesn't give that much."

Local police say the advent of money machines--which dole out cash any time, day or night--has brought with it a new type of crime with robbers and muggers devising a variety of schemes aimed at patrons of the automatic teller.

"Apparently, it's the wave of the future," says Capt. Thomas Novak of the D.C. Robbery squad.

"I'm trying to look towards the future and try to cope with it."

Other area police officials say that the number of reports of teller-machine holdups has varied since the devices were introduced to the Washington area in 1980. These officials would not say that the crime is widespread but they said they are watching out for a trend.

Members of about 1 million households in Washington, Maryland and Virginia have money cards that can be used at more than 110 area financial institutions. This fall, bank customers tied into the Network Exchange, the biggest local teller machine system, also will be able to handle money matters at machines installed at 151 grocery stores.

Shopping malls and gas stations eventually may be added to the list of places where automatic banking services are available, according to industry reports.

In the District, Maryland and Virginia, police say robbers have approached bank customers at night, forcing them at gun- or knifepoint to withdraw cash from checking or savings accounts. Other robbers have pounced on banking customers shortly after they have walked away from the machines with their freshly withdrawn cash. And a small number of area residents have also reported that they were abducted, taken to a teller machine and forced to withdraw money.

Banking officials say, however, that security measures, such as surveillance cameras and automatic photo equipment built into many machines, make after-hours banking at teller machines relatively safe. Some banks have drive-in automatic tellers, others have located their money machines inside bank entryways with doors that lock after a customer has gained entry with a money card.

"It's not really posing any major threat at all," said American Banking Association spokesman Sheldon Golub. "There's very little ATM automatic teller machine crime when you consider the person withdrawing money from a bank is just as likely to be mugged as a person who uses the machine."

But, police say, crimes against teller machine users, like most robberies, occur at night at sparsely populated locations.

The machines "provide so much bait, so much opportunity," explained D.C. Robbery Squad Capt. Thomas Novak, "because they contain so much money."

District police say there has been a recent rash of scams involving stolen bankcards in which robbers have called up the owner of the card, portrayed themselves as police officers and obtained from the cardholder the individualized, secret code necessary to start a machine transaction.

Earlier this year, banking customers in Washington and Maryland whose money cards were stolen or lost, were telephoned by persons identifying themselves as police officers. The fake officers asked for the personal identification number of the card holder and in some instances they got it.

"In a very short amount of time the maximum amount (allowed per transaction) was withdrawn," Novak said. In some cases, several transactions were made on one account until it was wiped out, Novak said. A suspect in one case was arrested last month.

Local police departments do not break down their robbery statistics to reflect crimes that occur in the vicinity of automatic teller machines, so police say they cannot pinpoint actual numbers of money machine holdups in their jurisdictions.

In some of the more dramatic cases reported by police, robbers have added a new twist to those situations where victims come up short of cash during a routine holdup. For example, last month, an Arlington man reported that a suspect with a gun approached him in a parking lot and, after discovering that he had a money card, forced him to drive to a nearby machine and withdraw $200.

District police say at least two residents have reported being similarly approached, abducted, taken to their bank and forced to withdraw money from their savings or checking accounts.

Still, area police say the most common crimes involving users of the 24-hour teller machines are those in which a suspect stakes out a machine, watching and waiting to rob late-night bank customers in need of quick cash.

District police say they have worked with some banks to improve the safety of after-hours banking. Some banks have relocated teller machines that were next to tall shrubs, walls or other barriers, which would camouflage robbers and their victims.

Golub, the spokesman for the banking association, contends the need to relocate a machine for safety is rare, however.

"One of the things banks do is to be cautious and smart in placing the machines," Golub said. Lighting, the nature of the area and how well-traveled the area is are examined for safety as well as marketing reasons, he said.

Police and banking officials agree that customers using the machines should take general precautions against crime such as avoiding unlit areas and going out alone at night. Knowing about safety measures taken at the teller machine sites can also help ward off would-be robbers.

For example, a 25-year-old woman using an automatic teller machine near the Ballston Metro station in Arlington last July was approached by a man armed with a knife who demanded she withdraw $100.

The woman told him that the two of them, the transaction, and their conversation, were being monitored by a camera looming over the machine.

The man quickly ran off into the night.