Argentine law requires that, prior to arrival in Argentina at any entry point, U.S. citizen tourist and business travelers pay a 160 USD reciprocity fee by credit card online at the Provincia Pagos website. There are English instructions here: http://www.migraciones.gov.ar/pdf_varios/reciprocidad/Online_payment_instructions.pdf#zoom=100 

Once paid, travelers must print out the receipt and present it to the Argentine immigration officer at the time of entry. The fee is valid for 10 years from the date of payment and for multiple entries. (So - how many of you kept your receipt from the 2013 ICANN meeting? Huh? You didn't? Fool.)

It is advisable to keep multiple copies of the receipt, as it must be presented every time you enter Argentina. The fee applies only to bearers of tourist passports. 

Most of this info taken from http://travel.state.gov/content/passports/english/country/argentina.html 

Except for the 'fool' part.

Milton L. Mueller