© 2002 by Walter de Gruyter GmbH & Co. KG, All rights reserved. This paper explores the role of aerodynamic factors in some assimilatory processes, focussing specifically on the assimilation of lingual fricatives to following tongue-tip trills in a variety of languages. It tests the hypothesis that the extensive anticipatory movements for the trill, directed to attain its highly constrained production requirements, override the aerodynamic conditions for frication. Articulatory, acoustic and aerodynamic analyses of sequences of fricatives plus trills were made for three Catalan speakers. Gradient and complete assimilation of lingual fricatives to trills was found in the articulatory data. Palatal fricatives proved to be more resistant to assimilation than alveolars. Phrasal boundaries proved to have an effect on fricative to trill assimilation. Aerodynamic analysis showed that trills are more constrained aerodynamically than voiced fricatives. The time required to achieve the pressure drop at the oral constriction for audible frication was in the range of 50 ms for voiced and 30 ms for voiceless fricatives. Hence, if onset of articulatory movements for the trill reach the articulator within 50 ms from onset of the movements for the voiced fricative, audible friction will not be achieved. Thus the early onset of movements for the trill bleed the postural and aerodynamic requirements to generate frication.