Nationals of the USA, Canada, most Western European countries, Australia and New Zealand do not need visas to visit Argentina, but check current regulations. Most foreigners receive a 90-day visa upon arrival.
To get yourself a 90-day extension, visit the Dirección Nacional de Migraciones (Immigration office; 4317-0200; Av Antártida Argentina 1355; 7:30am-1:30pm Mon-Fri). Set aside some time, as this pro-cess can take an hour or two. Strangely enough, overstaying your visa (AR$50) is cheaper than paying for an extension (AR$100), but it’s also much more stressful – and the rules can change quickly.
Another option if you’re staying more than three months is to cross into Colonia or Montevideo (both in Uruguay; Colonia can be an easy day trip) and return with a new three-month visa. This strategy is most sensible if you are from a country that does not require a visa to enter Uruguay.