The objective of the paper is to study complexity in the music of Mozart. A scheme of modelling the cognitive representation of music is proposed, based on the theories of Schenker and Komar. The hierarchical structures of these theories are expressed as networks of elaborations. Such networks are produced for each phrase of the exposition of the allegro moderato of Mozart's quintet K.452. The number of elaborations in each network is found to be constrained to a limited range, and when plotted against the number of notes in each phrase a weak negative correlation is found. This is taken as evidence that the cognitive demand on the listener is maintained at a more or less constant level throughout the movement. Complications concerning recurrent material and different tempi are discussed.