Rep. John B. Anderson (R-Ill.) left for a weekend trip to his district shortly before 7 a.m. yesterday. This time instead of having his wife drive him to National Airport, he took a bus to Dupont Circle, got on the subway and arrived at the airport at 7:30 a.m.

Not only was he in plenty of time for his 8 o'clock plane, but he met a constituent on the subway platform. "Hey, Congressman, I think you're doing a great job," the man said, uttering the words a congressman most likes to hear at 7:30 in the morning.

"Why thank you," Anderson said, as the two exchanged pleasantries about the Metro system. "As you can see I believe in conserving energy."

Anderson was one of thousands of people who passed through the National Airport subway stop during the opening day of Metro's Blue Line. As early as 7 a.m. people carrying bags could be seen debarking from the train on their way to catch a plane, a sure sign the airport stop was going to be used for the purpose for which it was intended.

As with the other stops along the line, there were hassles - the farecard machine broke down periodically, one of the down escalators called it quits early in the day, the shuttle bus service to the airport terminals was slightly erratic and many passengers said directions to the station from the terminal were not noticeable enough.

But most people seemed to be enchanted by their new toy, were understanding about the mix-ups and especially delighted with the view of the airport and Washington from the elevated platform. Four boys between the ages of 10 and 14 even traveled from the Potomac Avenue station near their Southeast Washington homes to the airport just "to watch the planes take off for awhile."

"Look at the sun rise over the Washington Monument. Isn't that beautiful?" said Taffy Swandby, a government worker who was one of a group fo Metro enthusiasts from Foggy Bottom Citizens Association who took the first train to National Airport.

Three bartenders from the Hawk and Dove on Capitol Hill stayed up all night in order to catch the first train from the Capital South station. (They missed it because the station didn't open until 6:30 as workers tried to find the right keys to unlock the gates).

Arriving at the airport, they went off for breakfast in the terminal and returned shocked at the prices. "I had scrambled eggs and it cost $4.73," said Bill Holdforth, who was wearing a T-shirt emblazoned with the slogan "I caught My Drift in El Secundo."

About 1,890 people passed through the gates into the airport Metro station before 1 p.m., and another 4,900 went out. Of these, about 1,000 used the free shuttle buses before 1 p.m., which take 10 minutes to circle the terminal if traffic is moving smoothly and 20 if it isn't.

The buses are supposed to arrive every five minutes: random observations showed they generally did. At least one Metro attendant, however, was not aware of the free shuttle service and gave inaccurate directions to travelers until she was informed.

The new traffic light to aid pedestrians crossing the heavily traveled airport road to the sidewalk leading to the terminals worked well. The most poorly designed feature of the station appears to be the elevator for the handicapped.

To reach it a handicapped person must travel well beyond the main entrance, cross two heavily traveled lanes without a traffic light and wait for assistance. The elevator was not operating yesterday.

Airport employees will probably not benefit much from the subway, said airport spokesman David Hess, because the first planes leave at 7 a.m. This means many employees must be on duty well before the subway opens at 6 a.m., and the second shift leaves after the Metro closes at 8 p.m.

Yesterday's riders included the expected mix of commuters, sightseers, Metro freaks, lunchtime joyriders and suitcase luggers aiming to catch a plane or just off one. The 10 bicycle lock-ups were taken early and half a dozen others were chained to the grate.

One of the most unimpressed visitors yesterday was Ardses Murdana who came from New York for the day on business. Being a New Yorker, he automatically asked directions to the subway. Did he realize yesterday was the first day of service? "No," he said.

Jay Spinner, a free-lance photographer, spent several hours in the airport station pursuing his hobby - taking pictures of Metro. He's been doing it for eight years and said he gives slide shows of his collection.

While he was in the Air Force, Spinner visited subway systems in London, Paris, Montreal and San Francisco, but Metro, he says, is "the ultimate."

"The concept is unique, the color scheme, the design . . . it's truly a work of art. Being an artist, it really turns my creative eye on."