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The present thesis investigated the relationship between rapid facial muscle reactions and emotionally relevant stimuli. In Study I, it was demonstrated that angry faces elicit increased Corrugator supercilii activity, whereas happy faces elicit increased Zygomaticus major activity, as early as within the first second after stimulus onset. In Study II, during the first second of exposure, pictures of snakes elicited more corrugator activity than pictures of flowers. However, this effect was apparent only for female participants. Study III showed that participants high as opposed to low in fear of snakes respond with increased corrugator activity, as well as increased autonomic activity, when exposed to pictures of snakes. In Study IV, participants high as opposed to low in speech anxiety responded with a larger difference in corrugator responding between angry and happy faces, and also with a larger difference in zygomatic responding between happy and angry faces, indicating that people high in speech anxiety have an exaggerated facial responsiveness to social stimuli. In summary, the present results show that the facial EMG technique is sensitive to detecting rapid emotional reactions to different emotionally relevant stimuli (human faces and snakes). Additionally, they demonstrate the existence of differences in rapid facial reactions among groups for which the emotional relevance of the stimuli can be considered to differ.