????Shares in the big global banks tumbled Friday. The deepest sell-offs were in Europe, where even the promise of a big bailout package hasn't quieted concerns over the future of the euro zone and bank exposure to stressed economies.

????But U.S. banks slumped too, as investors continue to fret over banks' exposure to problems in Europe and questions about how far the Senate will go to rein in bank profits.

????The biggest losers Friday were two Spanish banks that have sizable presences in the United States, Santander (STD) and Banco Bilbao (BBVA). Both were down 8% in New York trading Friday. Others hit hard in the sell-off were Barclays (BCS) of the U.K. and Deutsche Bank (DB) of Germany, both down 5%.

????Fears surrounding the fate of troubled European economies were evident in the market for credit default swaps, where the cost of insuring against a default by major government bond issuers soared. The cost of protecting against a default on five-year bonds rose 23% in Portugal, 17% in Ireland, 16% in Spain and Greece, and 10% in France, according to CMA.

????Because the big banks are dependent on the backstop of their national governments, the CDS spreads on big banks widened out as well, showing a rise in perceived risk. Santander's spread widened by 16% Friday, CMA data show, taking the price of insuring against a default back to last Friday's levels, before the European Union and International Monetary Fund rolled out a $1 trillion bailout plan.

????The U.S.-based banks were holding up a bit better, with the likes of Citi (C) and Bank of America (BAC) down 4%. BofA last week detailed its fairly limited exposure to the weak economies of the eurozone, so defaults there don't by themselves pose a huge danger.

????But the regulatory and legislative environment remains a risk in spite of the industry's massive lobbying power. The Senate's currently taking up a measure that would cap the fees banks can charge on automated teller transactions, which has been a dependable profit stream in recent years. And the fate of banks' lucrative derivatives businesses hangs in the balance as well.