The polls will open at 8 a.m. (1100 GMT) with results coming in from 9 p.m. (0000 GMT).
All candidates need a minimum 1.5% of the vote to be eligible for the general election, while the two main political blocs face internal leadership battles.
JUAN GRABOIS AND JUAN SCHIARETTIGrabois, 40, a lawyer and labor activist, will compete with Massa for main Peronist nomination but lags well behind in polls.
Schiaretti, 74, a regional governor, is a moderate with ties to Peronism running for a coalition of smaller parties.
Reuters Graphics Reuters GraphicsReporting by Maximilian Heath; Editing by Nicolas Misculin and Diane CraftOur Standards: The Thomson Reuters Trust Principles.
Persons:
SERGIO MASSA, Massa, PATRICIA BULLRICH Bullrich, Horacio Larreta, HORACIO LARRETA Larreta, Larreta, JAVIER MILEI, JUAN GRABOIS, JUAN SCHIARETTI Grabois, Maximilian Heath, Nicolas Misculin, Diane Craft
Organizations:
Peronist coalition's, la Patria, Peronist, Buenos Aires, Change, Bullrich, Harvard, Reuters Graphics Reuters, Thomson
Locations:
BUENOS AIRES, Buenos Aires, Argentina