Associated Transport Line of Houston and Panamanian Carriers Corp. of Panama are merging their services to South America this month.

The combination container, breakbulk and roll-on, roll-off service will double the frequency to the east coast of South America, to 10 days from 21 days formerly offered by Panamanian Carriers Corp., according to John Demere, ATL president.The combined service will also increase frequencies to 10 days on the west coast of South America from the biweekly service formerly provided separately by Associated Transport Line., Mr. Demere said.

''We merged the services to both coasts so we could increase the frequencies while continuing to call at all the same ports that we called before joining forces,'' Mr. Demere said. ''Now a shipper in the U.S. can book cargo from Houston, for example, to both Brazil and Chile with a single phone call.''

Shippers in the United States can move cargo bound for either coast with one-stop shopping via the Texpress Service and Mexican-based shippers can ship to both coasts with a single call to the Mexexpress Service, he said. ''We see this as a big plus for us because PCC is very strong in Mexico, and we're projecting strong export growth from Mexico to the East Coast South America,'' he said.

All six ships in the east coast of South America service and six ships in the west coast of South America service will call Houston, New Orleans and Altamira and Tampica, Mexico, when the service begins May 18, he said.

Three of the ships in each rotation are Astrahkan-type vessels with heavy lift roll-on, roll-off capacity and the three others are combo container-breakbulk ships that handle up to 150 20-foot containers, he said.

The service calls directly to Buenos Aires, Rio de Janeiro, Santos, Paranagua and San Francisco do Sol in Brazil, and directly to Esmeraldes and Guayaquil, Ecuador; Callao, Peru;Arica, Anafogasta, Valparaiso and San Antonio, Chile.