Recommended: register and take the MPRE early in your third year of law school (usually, the November before you will take the bar exam). That way, you can cross it off your list or will have another chance to take it the following March, if you don't pass the first time. (Even if you pass the bar exam, without the MPRE you will not be eligible for licensing until you do!)
The Multistate Professional Responsibility Examination (MPRE) is based on the law governing the conduct and discipline of lawyers and judges, including the disciplinary rules of professional conduct currently articulated in the American Bar Association (ABA) Model Rules of Professional Conduct, the ABA Model Code of Judicial Conduct, and controlling constitutional decisions and generally accepted principles established in leading federal and state cases and in procedural and evidentiary rules.
The MPRE is administered by the Law School Admission Council (LSAC) on behalf of the National Conference of Bar Examiners (NCBE). The MPRE is a 60 question, two-hour, multiple-choice examination administered three times per year at established test centers across the country.
Like the MBE, the MPRE is scored and scaled nationally. The Illinois Board of Admissions to the Bar requires a MPRE score of 80 or more. An applicant need not take the MPRE before taking the bar exam, but must have done so and obtained a passing score before he or she can be recommended for admission to the bar.
All but three United States jurisdictions require the MPRE for admission to their bar.
For full information, resources, access and faq's: http://www.ncbex.org/about-ncbe-exams/mpre/mpre-score-services/
From the website of the Illinois Board of Admissions to the Bar.
To register for the MPRE, click here. (You must first create an account with the NCBE.)