Americans, Canadians and Australians arriving at the airport in Argentina have been required to pay a "reciprocity fee" before being allowed into the country if they hadn't previously paid it. The fee is about $160 USD.
As announced by the National Immigration Authority, however, effective March 1, 2013, all American, Canadian and Australian passengers arriving into Argentina WITHOUT the printed receipt of the paid reciprocity fee will be returned to their point of departure. It will NOT be possible for the fee to be paid at any local airport or office any longer.
If you're planning on a visit, this is the link for registration and payment of the fee, before departure to Argentina:
Also, be sure to print out your payment receipt. Upon arrival in Argentina, this printed receipt must be presented at Immigration Control - the receipt will be scanned by the Immigration officials.