OBJECTIVE: The serotonin system has long been of interest in biological models of human personality. The purpose of this positron emission tomography (PET) study was to search for relationships between serotonin 5-HT1A receptor density and personality traits. METHOD: Fifteen normal male subjects, ages 20–45 years, were examined with PET and the radioligand [11C]WAY100635. Personality traits were assessed with the Swedish version of the Temperament and Character Inventory self-report questionnaire. Binding potential, an index for the density of available 5-HT1A receptors, was calculated for the dorsal raphe nuclei, the hippocampal formation, and the neocortex. For each region, correlation coefficients between 5-HT1A receptor binding potential and Temperament and Character Inventory personality dimensions were calculated and analyzed in two-tailed tests for significance. RESULTS: The authors found that the binding potential correlated inversely with scores for self-transcendence, a personality trait covering religious behavior and attitudes. No correlations were found for any of the other six Temperament and Character Inventory dimensions. The self-transcendence dimension consists of three distinct subscales, and further analysis showed that the subscale for spiritual acceptance correlated significantly with binding potential but not with the other two subscales. CONCLUSIONS: This finding in normal male subjects indicated that the serotonin system may serve as a biological basis for spiritual experiences. The authors speculated that the several-fold variability in 5-HT1A receptor density may explain why people vary greatly in spiritual zeal.