Pan American World Airways yesterday offered the lowest scheduled transatlantic air fare in history - $149 for a standby round trip Boston to Amsterdam - and the Civil Aeronautics Board took only a couple of hours to approve the plan.

The extraordinarily quick CBA approval allows Pan Am to begin offering the fare tomorrow. A one-way ticket to Amsterdam will cost $99.

Pan Am said the fares will remain at those levels for one month. Then, on July 15, the new permanent fares of $155 one way and $279 round trip will go into effect.

The airline will seat about 180 persons on the Boeing 707s that will leave Boston's Logan International Airport every day at 7:30 p.m. Passengers can buy tickets on a first-come, first served basis between 2 and 6:30 p.m. on the day of the flight. Open tickets (with no reservation) may be purchased in advance.

The flight is a "no-frills" operation. Although complimentary coffee and tea will be served, audio sets, soft drinks, and alcoholic beverages will have to be purchased during flight. Meals will be sold at the airport before departure, but not in flight.

Arrival time in Amsterdam is 8:15 a.m. the following day. The return flight leaves Amsterdam at 2 p.m. and arrives at Logan airport at 3:30 p.m.

The rapid CAB approval was prompted by the fact that Pan Am is about to lose its authority to fly from Boston to London tomorrow. Because of a new U.S.-British air agreement, only two cities were allowed to have direct American carrier flights to London, and on Friday the CAB chose New York and Los Angeles over Boston.

Rather than end overseas services from Boston, Pan Am decided instead to fly to Amsterdam beginning tomorrow, and the CAB approved the remarkably low fares as a one-month experiment to help the airline introduce its new service on such short notice.

Normally, the CAB considers financial impact statements on new proposed fares to insure that no airlines offer unrealistically low prices.

But in its request for quick approval in this case, Pan Am said, "in providing no profit impact figures, Pan American is relying on the board's willingness to accept carrier management judgement in the establishing of new, low, no-frill fares in a market that would be receiving non-stop, direct service for the time."

Pan Am spokesman Merie Richman said tickets may be purchased in conjunction with other published fares, so, for example, a flight from Washington to Boston to Amsterdam can be booked in Washington. But, he cautioned, the Boston-to-Amsterdam segment of the flight will have to be one a standby basis.

The normal fares effective after July 15 are still extremely competitive with other discount fares offered. Freddy Laker's Skytrain provides similar standby service from New York to London for a round-trip fare of $248.

Secretary of Transportation Brock Adams issued a statement yesterday commending Pan Am for inaugurating the new flight.

"This new no-frills service opens another low-cost gateway for the international traveler from the U.S. to the European continent," he said. Adams said the new fare is a product of "President Carter's policies of promoting competitive opportunities for the airlines."

Specially, Adams pointed out that a recently signed agreement between the U.S. and the Netherlands which was promoted by President Carter permitted Pan Am to apply for the discount fare.

In an unrelated action, the Justice Department urged the CAB to extend its open-entry experiment being conducted at Oakland, Calif., to the rest of the nation.

That experiment significantly reduces the time and expense needed for an airline to obtain approval to begin service between Oakland and other cities. All that must be shown by the entering airlines is evidence that the market can support a minimum level of service and that the airline applying is "fit."

Hugh Morrison Jr., acting assistant attorney general in charge of the Antitrust Division, praised the CAB for the experiment, but added that he could see no reason to limit it only to Oakland.

In still another action this week, the CAB voted to stop backing the International Air Transport Association, which has participated in setting international air fares. CAB Chairman Alfred Kahn, an advocate of deregulation, has been an outspoken critic of IATA price-fixing arrangements.