In response to a congressional request, GAO investigated: (1) the nature of discount pricing in the trucking industry and its effect on competition in the industry; and (2) the structure of the less-than-truckload (LTL) sector of the industry, changes to it since 1980, and possible causes of those changes. GAO also reviewed a tariff association's study on pricing. GAO did not find any conclusive evidence or legal cases involving predatory pricing in the trucking industry. GAO found that: (1) all regions showed an increase since 1980 in the market shares of the trucking industry's largest firms; (2) the increased market concentration in trucking was no higher than in manufacturing generally; and (3) the nationwide market share of the largest firms' LTL traffic increased from 25 percent in 1980 to 36 percent in 1984. GAO noted that tariff association data: (1) showed a pattern of excessively low prices within the central and southern regions that had the effect of driving small firms out of business; and (2) suggested that, while profits and rates in the central and southern regions may have been depressed relative to profits and rates in other regions during the late 1970's, this became less true during the 1980's.