The regulatory arm of the National Association of Securities Dealers is investigating Bank One Capital Markets, the bank's investment-banking unit, for possible violations stemming from systems-integration problems following the 1998 merger of the old Bank One Corp. and First Chicago NBD Corp., according to people familiar with the matter.

The investigation, sparked by a routine regulatory examination, may prove to be more of an acute embarrassment than a body blow to the beleaguered Chicago-based bank, the nation's fifth-largest in terms of assets, because it concerns internal-process problems, not the loss of customer funds.

Still, it once again raises troubling questions about how well the 1998 merger was accomplished. It comes as Bank One tries to leave behind its miserable recent experience and rebuild its uneasy relationship with investors and stock analysts. It recently named Jamie Dimon, former president of Citigroup, as its new chairman and chief executive. Its 1999 annual report stated, "We are taking the right steps to begin to reclaim investor confidence."

Indeed, the management shake-up at the bank continued Thursday, with the announced resignation, effective May 1, of Chief Financial Officer Robert Rosholt, 50 years old. "It is the right time for a change for me and for the corporation," Mr. Rosholt said in a statement.

Mr. Rosholt was the top financial executive during the bank's warnings in August and November that 1999 profit would fall short of expectations. The bank blamed the shortfall on shrinking growth and profit margins at its once-highflying credit-card unit, First USA. Investors were shocked by the warnings: Bank One's shares are down more than 40% since the summer. In 4 p.m. composite trading Thursday on the New York Stock Exchange, the shares stood at $31.4375, up 68.75 cents.