Abstract: 257 undergraduates in manipulated emotional states played computerised movies in which facial expressions of emotion changed into categorically different expressions. The participants' task was to detect the offset of the initial expression. An effect of emotional state was observed such that individuals in happy states saw the offset of happiness (changing into sadness) at an earlier point in the movies than did those in sad states. Similarly, sad condition participants detected the offset of a sad expression changing into a happy expression earlier than did happy condition participants. This result is consistent with a proposed role of facial mimicry in the perception of change in emotional expression. The results of a second experiment provide additional evidence for the mimicry account. The discussion focuses on the relationship between motor behaviour and perception.