BACKGROUND: As a prelude to future studies of subjects with different temperaments, we sought to develop a probe to measure differential amygdalar responses to novel versus familiar stimuli. Prior neuroimaging studies of the amygdala in humans to date have focused principally on responses to emotional stimuli, primarily aversive, rather than to novelty per se. METHODS: Eight normal subjects aged 22.4 +/- 1.3 years were scanned using functional magnetic resonance imaging (fMRI) during passive viewing of novel and familiar faces. RESULTS: Using this newly developed paradigm, we found greater fMRI blood oxygenation level dependent (BOLD) signal response within the right amygdala to novel versus familiar faces--all with neutral expression. Furthermore, although a new facial identity was always presented in the novel condition, signal in the amygdala declined over time as it did for the familiar condition. CONCLUSIONS: These results suggest that at least one primary function of the amygdala is to detect and process unexpected or unfamiliar events that have potential biological import, of which stimuli symbolic of fear or threat are but one possible example. We propose that this experimental paradigm will be useful for examining brain responses to novelty in different temperamental groups, as well as various psychiatric disorders.