" A medical terminological system (TS) is essentially an ontology consisting of concepts, attributes and relationships pertaining to medical terms. There are many TSs around today, most of which are essentially frame-based. Various efforts have been made to get a better understanding of the requirements and the conceptual and formal structures of TSs. However, the actual implementation of a TS consisted so far of ad hoc approaches starting from scratch and, due to ad hoc semantics of the representation, the interoperability with external applications of the knowledge represented is diminished. In recent years, Protégé has been gaining in popularity as a software environment for the development of knowledge-based systems. It provides an architecture for integrating frame-based ontologies with knowledge acquisition and other applications operating on these ontologies. In its recent version, Protégé provides the ability to specify meta-classes and -slots. This contributes to an explicit separability of knowledge levels and allows for an increased modeling flexibility. These properties, and the fact that it complies with a standard knowledge model, enable Protégé to be an attractive candidate for the implementation of frame-based TSs. This paper investigates how to specify a TS in Protégé and demonstrates this in a specific application in the domain of intensive care. Our approach is characterized by the utilization of a conceptual framework for understanding TSs and mapping its components onto Protégé constructs. This results in specifications of knowledge components for the implementation of terminological systems. The significance of our work stems from the generality of these specifications. This facilitates their reuse, leading to a principled process for the development of terminological systems for a broad spectrum of medical domains. "