The creation of new chemotherapeutic regimens that permit shortening the duration of treatment is a major priority for antituberculosis drug development. In this study, we used the murine model of experimental tuberculosis therapy to determine whether incorporation of the investigational new nitroimidazopyran PA-824 into the standard first-line regimen has the potential to shorten the 6-month duration of treatment. As demonstrated previously, PA-824 alone had significant bactericidal activity over the first 2 months of treatment. Moreover, the substitution of PA-824 for isoniazid led to significantly lower lung CFU counts after 2 months of treatment and to more rapid culture-negative conversion compared to the standard regimen of rifampin, isoniazid, and pyrazinamide. Despite this, there was no difference in the proportion of mice relapsing after completing 6 months of therapy (2 of 19 mice treated with PA-824 in place of isoniazid relapsed versus 0 of 46 mice treated with the standard regimen). Meanwhile, no other PA-824-containing regimen tested was superior to the standard regimen on any assessment. Thus, we were unable to establish a clear role for PA-824 in a treatment-shortening regimen that includes two or more of the current first-line drugs. Future preclinical studies should include the evaluation of novel combinations of PA-824 with new drug candidates in addition to existing antituberculosis drugs for their potential to substantially improve the treatment of both drug-susceptible and multidrug-resistant tuberculosis.