What are the aerodynamic conditions required for trills? To find out we had two subjects produce steady-state voiced, voiceless, and ejective alveolar trills. The backpressure during trills was intermittently bled with a tube of varying diameter (and thus impedance) inserted in the speaker’s mouth via the buccal sulcus and gap behind the back molars. Intraoral pressure was measured via a catheter inserted into the pharynx through the nose. The variation, impairment, or extinction of trilling as a function of gradual decrease in intraoral pressure was analyzed acoustically. It was found that (1) bleeding the oro-pharyngeal pressure by 2 cm H2O impaired sustained trilling; (2) the minimum Po required to sustain tongue-tip vibration is lower than that required to initiate it; (3) extinction (and reinitiation) of trilling generally results in a fricative; (4) the range of Po variation for trills is narrower than that for fricatives; (5) voiceless and ejective trills are significantly less affected by venting the backpressure than voiced trills. The behavior of trills in varying aerodynamic conditions accounts for observed phonological patterns: final trill devoicing, alternation between trills and fricatives, co-occurrence of trilling and frication, and limited distribution of trills. [Work supported by DGICYT, Spain, PB 96-1158, and by Committee on Research, UCB.]