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.