Changes in electromyographic discharge of 18 craniofacial muscles in young monkeys were assessed during the first 6 months of adaptation to nasal airway restriction. Thirteen experimental animals were paired with 13 controls for statistical analysis (Sign test, α < 0.05). The adapting craniofacial muscles were assessed for rhythmicity correlated with the respiratory pattern, and increase in spontaneous, background discharge defined as tonic activity. The following muscles were rhythmically active in normal monkeys: genioglossus (5 per cent of pooled observations from 13 animals), dorsal fibres of tongue (3 per cent), levator labii superiores (10 per cent), elevator of upper lip (13 per cent), dilator naris (68 per cent). In the experimental monkeys, significantly increased rhythmicity was demonstrated in: genioglossus (37 per cent, p = 0.001), dorsal tongue fibres (39 per cent, p = 0.004), lip elevator (43 per cent, p = 0.001). Rhythmic discharges were also induced in: geniohyoid (14 per cent, p = 0.031). digastric (7 per cent, p = 0.031), anterior temporalis (14 per cent, p = 0.031), lateral pterygoid (10 per cent, p = 0.031). Early adaptation to oral respiration rhythmically recruited muscles of tongue, upper lip-nasal region, and the mandibular depressors. Muscles of upper lip and nares showed maximum discharge at the start of inspiration; mandibular muscles exhibited a slow rise to maximum similar to that of the diaphragm. Muscles tonically active in the pooled observations from the controls were: geniohyoid (45 per cent), digastric (19 per cent), genioglossus (57 per cent), lip elevator (26 per cent), caninus (37 per cent), mentalis (63 per cent), anterior temporalis (53 per cent). Tonicity did not alter significantly in these 7 muscles, but was induced in several craniofacial muscles: the dorsal fibres of tongue (35 per cent, p = 0.031), superior orbicularis oris (25 per cent, p = 0.05), inferior orbicularis oris (32 per cent, p = 0.035), medial pterygoid (19 per cent, p = 0.031), lateral pterygoid (14 per cent, p = 0.008). Tonic activity increased by oral respiration was prevalent in tongue muscles and orbicularis oris.