In Venezuela, a country where power once came from the barrel of a gun, the ballot box is playing a growing role in the decision-making process.

Yesterday, for the first time, members of the ruling Democratic Action Party selected a presidential candidate in a direct primary. The winner by a wide margin was Luis Pinerua, 53, party secretary general.

He is favored to win next year's election and succeed President Carlos Andres Perez. The president, believed to favor a second candidate, Jaime Lusinchi, 53, remained neutral in the primary campaign and did not vote.

Venezuela has been ruled by elected civilian presidents since 1958, but in the past, presidential candidates of both major parties have been chosen by party leaders at a convention. The decision was considered too difficult for the rank and file.

The primary was conducted amiably. Lusinchi, the party's congressional leader, is a long-time friend of Pinerua, and the two often spend weekends at adjoining beach houses outside Caracas.

There were few policy differences between the two men, but each has a personal following within the party that threatened to cause a split. A dispute within the party was blamed for Democratic Action's close loss in the 1968 presidential election.

This year, both candidates pledged in advance to accept the primary's result.

While Venezuela is more democratic than most of its South American neighbors, there were doubts that a direct primary could be conducted successfully. The idea of conducting party business in public made the party more than a little nervous.

"You have to understand," said one government official, "that it amounts to an admission that there are differing opinions in the party," a situation that is known but not generally mentioned aloud.

The opposition Social Christian Party, which has won only one presidential election since democracy was restored 20 years ago, has no plans to conduct a primary. "Its leader, former president Rafael Caldera, denigrated yesterday's primary as pandering to the public's love of rhetoric.

Despite the success of the primary, there is no guarantee that it is a precedent.Democratic Action President Gonzalo Barriors said the primary may not become a permanent political fixture here.

Former ambassador to Washington antique Tejera Parks and other party leaders, however, believe that the system will be institutionalized.

Some political observers said the decision to hold a primary was made by regional party leaders who feared losing their jobs if they supported a losing candidate in a convention battle.