Thomas Cook & Son, the century-old company whose name was once synonymous with tours from Abyssinia to Zanzibar, is hitching a new wagon to its caravan of services.

Its North American affiliate, Thomas Cook Inc., last week launched wholesale foreign currency operations in New York. Cook will buy and sell all foreign currencies, from the pound sterling to the high-risk Nicaraguan cordoba, which the new government of that country recently recalled.

Its chief customers will be American banks, particularly the 4,000 or more that sell Cook's Travelers Cheques. Its chief competitors will be established dealers such as Deak-Perera, Republic National Bank and Manfra, Tordella & Brookes. Deak, the largest, exchanges more than $2 billion in currency annually. Unlike Deak, Cook won't offer its service to the general public.

"We expect to give them a good run for the money," said Joe Chernowski, director of the foreign currency exchange division, adding that the company was just formalizing what it has been doing informally for more than 100 years. The first Cook tour took place in 1845 and, by 1874, the company was issuing its Travelers Cheques and changing money for far-flung Englishment and others.

Cook has been operating currency exchange offices in London for 15 years and in Paris for a decade. Chernowski said this was a propitious time to establish an exchange here because "there is a heightened awareness (by Americans) of foreign exchange dealings now that the dollar is not the almighty instrument it used to be."

Nevertheless, he expects more transactions will involve travel, with only a negligible number for investment purposes.

Foreign tourists, moreover, are not as "plastic-oriented" as Americans, he continued. So Cook intends to provide an outlet for U.S. banks to sell their excess foreign currencies.

The new service will operate out of Thomas Cook headquarters in New York. Transactions can be made on a toll-free line at a guaranteed rate on the spot.