The authors have developed a speech synthesis system based on articulation in order to study relationships between the human vocal tract shapes and speech signals. The articulatory speech synthesizer proposed by Sondhi and Schroeter is used to produce speech sounds in the system. By using a GUI (Graphical User Interface) technique, the system can provide easy and interactive operation for users. Therefore, the system is useful not only for speech research but also for teaching materials in speech science. In this paper, effectiveness of the system for understanding articulatory-to-acoustic relationships is shown for steady vowels. Modification of the system for producing continuous vowels is also shown with more realistic description of vocal tract shapes.