Dimon said "the financial system is safer today than it was in '07," but he was hesitant to say that was due to regulations.
When Sen. Bob Corker, R-Tenn., pressed Dimon if regulatory systems have made systems safer, Dimon said, "I don't know."
In his prepared remarks, made available before the hearing, Dimon defended JPMorgan's performance, even while admitting that the bank felt "terrible" for having lost some of its shareholders' money.
"We will lose some of our shareholders' money - and for that, we feel terrible - but no client, customer or taxpayer money was impacted by this incident," he said.
Though Dimon since has castigated himself publicly for having misjudged the situation, shareholders, analysts and politicians have said they cannot understand how the normally indefatigable Dimon could have let a mistake this size occur.
Dimon insists that the full magnitude of the London trading risk was not known to him, and that he remained ignorant of key details until it was too late to prevent the loss. That surprises one of his colleagues from the 1990s, who thinks of Dimon as the ultimate detail guy: "What surprises me most in this whole debacle is that he really was a detail guy. Nothing slipped by him. My guess is he may have delegated too much and not have known. If he had, I don't see how it could have reached this level of loss."
Dimon was pressed by the Senators on a number of other issues, including mortgages and investors who have not received their money back from bankrupt MF Global.
Sen. Jon Tester, D-Mont., said about 100 of his constituents are farmers who have not received their hedged money back from MF Global, which inappropriately transferred client funds through JPMorgan.
Tester said "there were a lot of farmers that hedged to protect themselves from bad outcomes."
"I hope they get all their money back," Dimon said of Tester's constituents. "I still believe they will by the way."
ABC News' Alan Farnham and Sunlen Miller contributed to this report.