The institution of special trade relations, such as in preferential-trade areas (PTAs)requires some evidence of the origin of the products that may benefit from theconcessions. These conditions constitute the so-called rules of origin. These rules aredesigned to preclude the exports from outside countries to the PTAs through thecountry-member with the lowest tariffs. However, like other measures applied in theinternational trade, the rules of origin can be used as non-tariff barriers and conferhigher levels of protection to some sectors.The main purpose of this paper is to analyze the regimes of rules of originadopted by Nafta, Mercosur and that used by the European Union in its GeneralSystem of Preferences (GSP). In addition, it will be investigated to what extent the?hidden? protection provided by the use of rules of origin can be associated withdifferences in the tariff rates. The empirical results suggest that the incidence of morerigorous criteria to confer origin is easily detected in some sectors considered?sensitive? in the developed countries, such as agricultural products, food, textiles andclothing and footwear.