Please check this reference from the Organization of American States' site:


"For the purposes of this Convention the terms "exhortos" and "cartas rogatorias" are synonymous in the Spanish text. The terms "letters rogatory", '"commissions rogatoires", and "cartas rogatórias" used in the English, French and Portuguese texts, respectively, cover both "exhortos" and "cartas rogatorias""

(http://www.oas.org/juridico/english/Treaties/b-36.html)



and EuroDicAutom:

Spanish Term comisión rogatoria
Reference Base de Terminología Judit,Tribunal de Justicia de las CE;
Reglamento de Procedimiento del Tribunal de Justicia de las Comunidades Europeas de 19 de junio de 1991(DOCE L 176,1991,32/1)
English Term letters rogatory
Reference Judit Terminology Data Base,Court of Justice of the EC;
Rules of Procedure of the Court of Justice of the European Communities of 19 June 1991(OJEC L 176,1991,32/1)



Good luck!