A sensitive and precise LC-ESI-MS/MS method for determination of nutlin-3a in murine plasma using ketoconazole as an internal standard was developed and validated. Plasma nutlin-3a samples were prepared by either a simple protein precipitation (PP) for the high concentration range (10-20,000ng/mL) or by liquid-liquid extraction (LLE) for the low concentration range (0.25-300ng/mL). Nutlin-3a and ketoconazole were separated on a modified C18 analytical column (4microm, 75mmx2mm) with an isocratic mobile phase (acetonitrile/5mM HCOONH(4)=70/30, v/v). The retention times of nutlin-3a and ketoconazole were 1.14 and 1.45min. Detection was achieved by a tandem MS system, monitoring m/z 582/99 and m/z 532/82 for nutlin-3a and ketoconazole, respectively. The PP method was linear in a range of 10-20,000ng/mL (R(2)>or=0.993) and the LLE method was linear in a range of 0.25-300ng/mL (R(2)>or=0.992). The mean recoveries for PP and LLE were 24% and 78%, respectively. Within-day and between-day precisions were <or=4.5% for PP and were <or=4.9% for LLE. Within-day and between-day accuracies (% error) ranged from 4.8 to -7.9 for PP, and from -0.2 to -8.4 for LLE. The two extraction methods produced equivalent results, allowing use of both within the same study. This method has been applied to the measurement of nutlin-3a concentrations in murine plasma samples obtained from a preclinical pharmacokinetic study.