In order to investigate both the relationship between perioral muscular activity and tooth position and also the clinical indicators of this activity, two quantitative EMG studies were carried out. The first investigated the reproducibility of the method in 16 adults. Subsequently, a study employing the same protocol was conducted on 41 subjects with untreated malocclusions. Activity was measured during speech, whilst chewing biscuit, swallowing saliva, resting, and in those subjects with incompetent lips, during lip-seal. In addition to the EMG recordings, measurements were taken from study models, lateral skull radiographs and of the clinical lip heights. In general between visits random variability was sufficiently small for the method to be applied experimentally. The results suggest that the effects of perioral muscular activity are determined by the presence or absence of competent lips. This was despite similar levels of activity in both groups except during chewing. Resting activity did influence incisor position in subjects with competent lips but the intermittent activity from chewing, speaking and swallowing, did not. No clinical indicators of the level of perioral activity were found.