Argentinian pesos is the official currency in Argentina and despite what some travellers think, US dollars are not commonly used within the country. Prices for big-ticket items like property, cars, and international travel are often quoted in US dollars but payment is usually expected in pesos.
If you're visiting Argentina, then you're probably best to use the cash machines to get out local currency. Do bring some "hard" currency with you such as USD or EUROS because you'll need some for the first day or two until you get to a cash machine (there's one at the airport but doesn't work 100% of the time with foreign cards.)
For currency exchange, try one of the banks or currency houses on the Calle Florida or nearby streest in central Buenos Aires.
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Living in Argentina Blog