Since late 1995 turnover at the CEO level of some of the largest firms in the U.S. has reached dramatic proportions. The Center for Executive Options, a unit of Drake Beam Morin Inc. in Boston says that 31 CEOs from Fortunes list of top 100 companies have been replaced in that short two-and-one-half year period. Among the reasons: lack of performance, mergers, acquisitions, and corporate restructuring. And CEO longevity at those companies is getting shorter, with the median tenure now four years, says Jerry Martin, president of Drake Beams Center for Executive Options. The largest disruptions, he says, have been in the telecommunications, financial services, and insurance industries.