Various issues and difficulties in building a prototype semantic network for representing a metathesaurus of concepts for the Unified Medical Language System (UMLS) are identified. The use of several methods to overcome these difficulties is explored. The best approach appears to be a combination of the top-down and bottom-up methods. The bottom-up approach is useful for exploring the information in a given domain, and for testing the robustness of the representation by attempting to fit instances of concepts into their corresponding generic frames. However, when used alone this approach tends to get bogged down with detail. The top-down approach of conceptual modeling is useful because it helps maintain consistency, and keeps the entire semantic network in perspective by hiding detail.