The synthetic opioid peptide analog Tyr‐D‐Ala‐Gly‐N‐methyl‐Phe‐Gly‐ol (DAMGO), which is a mu opioid receptor‐selective agonist, was quantified in ovine plasma samples with matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS), using delayed extraction and a reflectron. The internal standard was pentadeuterated DAMGO. Timed‐ion selection was used to select the precursor ion. The analysis of the post‐source decay fragments improved the detection sensitivity, and the use of the precursor–product ion relationship optimized the specificity. For plasma samples, the inter‐assay variability of this method was 6.4% (n = 79) and the intra‐assay variability was 6.0% (n = 10). The variability for controls was 3.4% (n = 43). The profile of DAMGO amount versus time was determined in sheep plasma, and the corresponding pharmacokinetic data were calculated. Copyright © 2000 John Wiley & Sons, Ltd.