COMPOST is the result of our first reflections on the computing requirements for synthetic speech generation. It combines the basic facilities of a rule-oriented software with object-oriented design. COMPOST is a tree compiler: it manipulates and transforms not only lists of atoms but also trees and subtrees. Basic atoms involved into the tree inherit of properties of user defined classes . Features and numerical cues are associated with each atom. COMPOST has then numerical capabilities to do synthesis-by- rule. COMPOST has been developped to build a formant-based rule system but may generate any king of parametric trajectories and then could be used in rule-based articulatory synthesis.