The diplomatic pouch is one of those ideas that work better in principle than in practice. The privilege of sending sealed pouches through customs is essential for all embassies. The abuse of the practice is probably inevitable. And now, the temptation to crack down on abuses threatens to cause more trouble than it's worth.

Rep. Stephen J. Solarz, D-N.Y., has introduced a bill that would direct the president to adopt measures to combat misuse.

The concept of the pouch is almost as old as the word "diplomacy" which, from its Greek origin, means "a letter folded so that its contents cannot be read." The diplomatic pouch became part of the lexion of international law at the Congress of Vienna in 1815, and was redefined in the 1961 Vienna Convention.

Recognizing the need for the inviolability of instructions, secret documents, and other materials, the 1961 Convention states that the diplomatic pouch shall not be opened or detained. If a country has reason to believe a pouch contains contraband, it may ask a member of the mission receiving the pouch to open it in the presence of customs officials. If the mission refuses, the pouch will be sent back to the originating country, unopened.

Human nature being what it is, there have been abuses. There is no size limitation to what constitutes a "pouch." The Soviets once tried to send a sealed, 9-ton truck into Switzerland, claiming the entire vehicle was a diplomatic pouch.

After a coup in Nigeria, British customs officials discovered that a crate on the way to Lagos that Nigerian diplomats had sealed as a diplomatic pouch containing a sedated official of the former regime wanted for trial back home.

Abuses notwithstanding, the ability to send materials in and out of a country is of prime importance to all diplomatic missions, none more so than the United States, by far the largest sender and receiver of pouches.

Some 42,446 pouches were sent from Washington by the American diplomatic corps during a nine-month period last year.

Any attempt to alter unilaterally what has become accepted international law would surely leave the United States more vulnerable to reciprocal retaliatory action - another time-honored practice in diplomacy.