MEYER, J. D., K. M. CROMBIE, D. B. COOK, C. J. HILLARD, and K. F. KOLTYN. Serum Endocannabinoid and Mood Changes after Exercise in Major Depressive Disorder. Med. Sci. Sports Exerc., Vol. 51, No. 9, pp. 1909–1917, 2019. The endocannabinoid (eCB) system is implicated in the pathophysiology of depression and is responsive to acute exercise in healthy adults. Purpose: We aimed to describe acute changes in serum eCB across a prescribed moderate (MOD) and a self-selected/preferred (PREF) intensity exercise session in women with major depressive disorder (MDD) and determine relationships between changes in eCB and mood states. Methods: Women with MDD (n = 17) exercised in separate sessions for 20 min on a cycle ergometer at both MOD or PREF in a within-subjects design. Blood was drawn before and within 10 min after exercise. Serum concentrations of eCB (anandamide [AEA], 2-arachidonoylglycerol) and related lipids (palmitoylethanolamine, oleoylethanolamine, 2-oleoylglycerol) were quantified using stable isotope-dilution, liquid chromatography/mass spectrometry/mass spectrometry. The profile of mood states and state-trait anxiety inventory (state only) were completed before, 10 min and 30 min postexercise. Results: Significant elevations in AEA (P = 0.013) and oleoylethanolamine (P = 0.024) occurred for MOD (moderate effect sizes: Cohen’s d = 0.58 and 0.41, respectively). Significant (P < 0.05) moderate negative associations existed between changes in AEA and mood states for MOD at 10 min (depression, confusion, fatigue, total mood disturbance [TMD] and state anxiety) and 30 min postexercise (confusion, TMD and state anxiety). Significant (P < 0.05) moderate negative associations existed between 2-arachidonoylglycerol and mood states at 10 min (depression and confusion) and 30 min postexercise (confusion and TMD). Changes in eCB or related lipids or eCB–mood relationships were not found for PREF. Conclusion: Given the broad, moderate–strength relationships between improvements in mood states and eCB increases after MOD, it is plausible that the eCB system contributes to the mood-enhancing effects of prescribed acute exercise in MDD. Alternative mechanisms are likely involved in the positive mood state effects of preferred exercise