This paper discusses effects of supra-lexical linguistic rhythm on the appearance or absence of optional schwa. Specifically, the roles of rhythmic alternation and prosodic parallelism are studied in three experiments and weighed against each other. In Experiment One, an oral reading study, readers were confronted with either of the two graphemic representations of the alternating adverb <gern(e)> (‘happily’) in sentential contexts the rhythmic structure of which was systematically varied. The evaluation of the scripted speech productions suggests that readers take the rhythmic environment into account when choosing an allomorph for the prosodically variable target word. Experiment Two is concerned with prosodic determinants for the morphosyntactic alternation in German partitive or possessive constructions. These may be realised as genitive attributes or using a prepositional construction. A forced choice experiment with written material suggests that participants consider the distribution of strong and weak syllables when choosing among the morphosyntactic variants. Experiment Three exploits the prosodic alternation of four adverbs. Analysing the distribution of the variants in a large written corpus attests that the immediate prosodic context affects the choice among the variants. A synopsis of the findings suggests that rhythmic alternation (conceived as the joint effects of stress clash avoidance and stress lapse avoidance) has a stronger impact on the presence or absence of a reduced syllable compared to prosodic parallelism.