This paper outlines a constructivist theory of learning with a more concrete and specific approach to the types of problems addressed in Piaget's equilibration theory. Knowledge, in this constructivist view, results from the subject's transformational activity on the world and his/her real efforts to organize that activity. The first section introduces fundamental implications of the constructivist position which are considered necessary to models of learning mechanisms. Discussion focuses on (1) the construction of knowledge through reflection on praxis in relation to the real world; (2) the nature of the interactive relation between organism and environment; and (3) the person as the primary causal agent in the construction of knowledge. Expanding Piaget's biological metaphor in the direction of systems theory, the second section proposes a constructivist interpretation of the composition of mental models. The discussion leads to the specification of aspects of a mechanism of coordination resulting in the establishment of a new internal control structure in the individual by which actions are "crystallized" into meaning. The final section presents a model of the mechanism of change in representational capacities viewed as action-based models. A three-page list of references concludes the document. (RH)