In 1981, EIA began a major study of the impact of natural gas deregulation. Through 1981, the major product of that study was the August 1981 EIA analysis paper entitled ''Analysis of Economic Effects of Accelerated Deregulation of Natural Gas Prices.'' That paper will be referred to as the ''Deregulation Study'' below. The Natural Gas Market Model (NGMM) was the primary model used to produce the forecasts discussed in the Deregulation Study. A modified version of NGMM has been used in the initial runs of the EIA Extended Short-term Forecasting System (ESFS), which is still under development. The purpose of this paper is to provide a comprehensive description of what NGMM is, and of the inputs used with NGMM for the Deregulation Study. The Deregulation Study, and the many documentation reports it cites, contain much information about the substantive studies which led up to the forecasts; however, it does not provide enough detail on how these studies were brought together to permit either a replication or an in-depth evaluation of the forecasts. EIA standards require that models be documented in enough detail to permit replication. This report attempts to fill that gap in documentation, on the basis of a line-by-linemore » audit of the model code, interviews with the model developers, and a replication of the model in the user-oriented system Troll. The report mentions the mechanics of how the solutions are obtained, but not in complete detail. 2 figs., 6 tabs.« less