To facilitate the integration of terminologies into applications, various terminology services application programming interfaces (API) have been developed in the recent past. In this study, three publicly available terminology services API, RxNav, UMLSKS and LexBIG, are compared and functionally evaluated with respect to the retrieval of information from one biomedical terminology, RxNorm, to which all three services provide access. A list of queries is established covering a wide spectrum of terminology services functionalities such as finding RxNorm concepts by their name, or navigating different types of relationships. Test data were generated from the RxNorm dataset to evaluate the implementation of the functionalities in the three API. The results revealed issues with various aspects of the API implementation (eg, handling of obsolete terms by LexBIG) and documentation (eg, navigational paths used in RxNav) that were subsequently addressed by the development teams of the three API investigated. Knowledge about such discrepancies helps inform the choice of an API for a given use case.