Little is known regarding the relationship between how mood is experienced and biochemical factors. This research has explored potential relationships between whole-blood serotonin (5-HT) and subjective mood, measured using the Positive and Negative Affect Schedule (PANAS), in healthy males (n = 23). Univariate linear regression analysis indicated that 5-HT levels were positively related to positive affect (P = 0.002) but not negative affect. Whole-blood 5-HT concentrations therefore appear to predict mood in healthy males. More research using larger samples and including females is needed to explore further associations between biomarkers and subjective measures of mood. (c) 2005 Elsevier B.V. All rights reserved.