Judges for a federal appeals court sharply questioned whether regulators worried about market fairness can require stock exchanges to experiment with the fees they charge.

The judges, hearing a lawsuit Friday brought by the New York Stock Exchange and other trading venues, debated whether federal regulators justified a pilot programme that would limit the trading fees charged by exchanges. At issue is the dominant pricing system used by exchanges, which charges fees for some trades while giving brokers subsidies for sending certain other orders to their venues.