Federal trade regulators are finally getting around to their long-promised review of the anti-trust regulations they use when evaluating the competitive effects of so-called “horizontal mergers” between two business rivals.

The Horizontal Merger Guidelines, as they are known, are used by the Federal Trade Commission and the Justice Department to determine whether such deals pass muster with U.S. anti-trust laws. They also convey the agencies' general views on anti-trust policy and enforcement, and as such are much-watched by businesses and the courts.

The proposed revisions, published last month and out for comment until June 4, are the culmination of regulatory spadework the ...