The New York Stock Exchange (NYSE) has been asked how it calculates its pay package for chief executive Richard Grasso by the US financial watchdog, the Securities and Exchange Commission (SEC).
Mr Grasso's pay is under focus
The move follows last week's announcement that the NYSE had paid Mr Grasso nearly $140m (£89m) in savings, benefits and incentives that he had built up during his time at the exchange.
The exchange also said Mr Grasso's contract had been extended by two years to 2007 and he would receive basic pay of $1.4m and an annual bonus of at least $1m.
The announcement attracted controversy, with many observers saying the size of Mr Grasso's pay package appeared excessive.
The SEC is now asking the NYSE for "full and complete" details on how the pay package is determined.
SEC chairman William Donaldson - himself a previous head of the NYSE - has sent a letter to H. Carl McCall, the head of the exchange's Human Resources and Compensation committee.
In the letter, Mr Donaldson says the approval of Mr Grasso's pay package "raises serious questions regarding the effectiveness of the NYSE's current governance structure".
The letter sets out a number of questions and gives the NYSE until 9 September to reply.
NYSE spokesman Rich Adamonis said Mr McCall was "in receipt of Mr Donaldson's letter for additional information and he will respond by the September 9 date".
The $139.5m one-off payment to Mr Grasso announced last week consisted of $40m from a savings plan, accrued retirement benefits of $51.6m and $47.9m relating to prior incentive awards.
Mr Grasso had said he wanted to withdraw the benefits "in order to facilitate personal financial and estate planning".
Earlier this year the NYSE said it would disclose how much it paid top executives in order to improve its corporate governance standards.
Mr Grasso's pay had come under attention after reports had put his total pay package at $10m a year.