Violeta Chamorro, the widow of the man whose assassination played a large role in setting off the rebellion against dictator Anastasio Somoza, resigned today from the revolutionary junta ruling this Central American nation.

In her letter of resignation, Chamorro, 50, said a back ailment has made it difficult for her to keep up with the heavy schedule followed by all five junta members.

She said she hoped her resignation would not be interpreted as a political act "since I support the process of change that we have been carrying out."

Chamorro, whose family owns the powerful independent newspaper La Prensa, is one of two members of the junta considered representative of the business community and middle class. The other three members belong to the Sandinista Front that overthrew Somoza and says it is carrying out a revolution for Nicaragua's peasants and workers.

Informed sources said a new member of the junta probably would be names since a four-member group could split on some issues.

The present Junta was named by the Sandinista Front's nine-member National Directorate, which also participates in all major government decisions.

Although Chamorro said her resignation was not for political reasons, it adds some uncertainty to the political scene here.

There is discontent among the middle class and business people, who think the Sandinistas are moving too fast to change the country's economic structure. If the Sandinistas wish to reassure this group, they will name another representative of what they call the bourgeoisie to the junta.

Chamorro had not been active politically before her husband Pedro Joaquin, a political enemy of Somoza, was assassinated Jan. 10, 1978. The murder set off an outpouring of protest that led to the insurrection that brought Somoza down.

She and the other members of the junta were named to their posts while in exile before Somoza resigned. She flew to Nicarague in a small plane, the night before the Sandinistas marched into Managua.

As a member of the junta, Chamorro has represented Nicaragua on several trips abroad.

Chamorro's two sons are carrying on the family's journalistic tradition on opposite sides of the political fence. Pedro Joaquin Chamorro Jr. is the most outspoken press crtici of the government, while his younger brother Carlos is editor of the Sandinista newspaper Barricada.