Pursuant to a congressional request, GAO reviewed the Highway Trust Fund, focusing on the: (1) sources and amounts of trust fund revenues generated between fiscal year (FY) 1987 and FY 1991 and their uses; (2) estimated remaining Fund balance when federal-aid highway and mass transit programs expire at the end of FY 1991; and (3) influence that recent legislation could have on surface transportation spending during the next authorization period. GAO found that: (1) federal highway user taxes on motor fuels, tires, and trucks support the Fund; (2) during FY 1987 through FY 1991, those taxes generated about $73.46 billion, with about 64 percent of those revenues generated through gasoline taxes; (3) while the majority of highway tax revenues are credited to the Fund for highway and mass transit projects or related activities, exceptions included cleanups of petroleum-contaminated groundwater and general fund contributions for use in deficit reduction; (4) the Fund will have an estimated uncommitted balance totalling $17.7 billion dollars by the end of FY 1991 if all outstanding commitments authorized through FY 1991 for both highway and mass transit programs are liquidated using the existing cash balance plus the allowed future revenues; (5) Department of Transportation officials recommended a $3.5-billion safety cushion to guard against inaccurate revenue projections, which could reduce the estimated uncommitted available funds to $14.2 billion; (6) recent legislation setting annual discretionary spending limits in separate defense, international, and domestic categories will determine, in part, the actual spending limits for federal-aid highway and mass transit programs through FY 1995; and (7) in FY 1994 and FY 1995, there will be a single cap on all discretionary spending, and highway and mass transit programs will have to compete with all discretionary programs for available funds.