A rapid and sensitive ultra performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) method has been developed and validated for the quantitative determination of sunitinib and its n-desethyl metabolite SU12662, in 100microl aliquots of human potassium EDTA plasma with deuterated sunitinib as internal standard. As sunitinib was found to be extremely sensitive to light causing rapid conversion of the Z (cis)-isomer to the E (trans)-isomer, the sample extraction and cleaning-up were performed under sodium-light and in amber vials. The extraction involved a simple liquid-liquid extraction with tert-butyl methyl ether. Chromatographic separations were achieved on an Aquity UPLC((R)) BEH C(18) 1.7microm, 2.1mmx50mm column eluted at a flow rate of 0.250ml/min on a gradient of acetonitrile. The overall cycle time of the method was 4min, with elution times of 1.05, 1.43, 0.95, and 1.34min, for the E (trans)- and Z (cis)-isomers of sunitinib and the E (trans)- and Z (cis)-isomers of SU12662, respectively. The multiple reaction monitoring transitions were set at 399>326 (m/z), at 371>283 (m/z) and at 409>326 (m/z) for sunitinib, SU12662 and the internal standard, respectively. The calibration curves were linear over the range of 0.200 to 50.0ng/ml with the lower limit of quantitation validated at 0.200ng/ml for both sunitinib and SU12662. The within-run and between-run precisions were within 11.7%, while the accuracy ranged from 90.5 to 106.8%.