This information was provided to BAIN from the US Embassy website. Please read on for new regulations requiring the reciprocity fee for entering Argentina from the USA:

Effective October 31, 2012 (Aeroparque) and December 28, 2012 (Ezeiza)
The National Immigration Agency (Dirección Nacional de Migraciones) has
added a new form of payment of the reciprocity rate (visitor visa – rate for
Americans of USD$160), through the Provincia Payment System. Until now, this
fee was paid upon arrival at the airport (either Aeroparque or Ezeiza). Effective
October 31, 2012 for arrivals to Aeroparque and effective December 28, 2012 for
arrivals to Ezeiza International airport, all U.S. tourist or business visitors must pay
the reciprocity rates (USD$160 for Americans) with their credit card through the
on-line system. After these dates, cash payments will NOT be accepted at the
airports.
How to pay the reciprocity fee on-line:

1. Enter the web site http://www.migraciones.gov.ar or http://www.provinciapagos.com.ar of Provincia Pagos and register to start the process.
2. Complete the form with the corresponding personal and credit card information.
3. Print the payment receipt.
4. On arrival in Argentina, this printed receipt must be presented at Immigration Control. The receipt will be scanned by the Immigration officials, the information will be checked, and the traveler’s entry to the country registered.