Me parece así de simple...

Spanish term (edited): ley de regimen tributario interno

http://www.google.com/search?hl=en&client=firefox-a&rls=org....

See:
"amending the "Regulations for the Application of the Internal Tax Regime" (Reglamento para la Aplicación de la Ley de Régimen Tributario Interno-RALRTI)"

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Note added at 15 hrs (2009-07-12 16:35:10 GMT)
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Google books with "internal tax regime" references:

A book from 1989:
http://books.google.com/books?id=s1NEdqvtd3IC&pg=RA1-PA198&l...

"The United States guarantees do not extend to taxes imposed by Puerto Rico, which adopted its own internal tax regime pursuant to the ..."

A book from 2007:
http://books.google.com/books?id=yT1WEw-yJzkC&pg=PT481&lpg=P...

"Under the latest amendments to the Law on the Internal Tax Regime, every transfer of goods or provision of services is subject to VAT. ..."

Another reference:
http://www.aals.org/international2004/Papers/Simmons.pdf

"For any practitioner, obtaining information about the internal tax regime of the country into which an investment is targeted is one of the more difficult issues of transnational commercial planning"

A paper from the Harvard Law School:
http://74.125.47.132/search?q=cache:w-Q7dGKRGPwJ:www.law.har...

"When Chile modified its internal tax regime in light of the Japan-Alcoholic Beverages decision, the EC brought a new complaint, alleging that the modified law still violated the anti-discrimination rules of GATT Article III. After the Appellate Body found against it, Chile lowered its taxes on all distilled spirits to the same level applied to pisco."

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