All bills for raising revenue shall originate in the House of Representatives; but the Senate may propose or concur with amendments as on other bills.

From The New York Times:

The House jealously guards its power to originate tax bills. The Constitution says, “All bills for raising revenue shall originate in the House of Representatives; but the Senate may propose or concur with amendments as on other bills.”

The Senate version of the bailout package is technically an amendment to a House bill that would require group health insurance plans to provide equivalence, or parity, in the coverage of mental and physical illnesses.

Insurers often charge higher co-payments and set stricter limits for mental health care than for the treatment of physical illnesses. The bill would outlaw such discrimination, and the government could impose an excise tax on health plans that violated the new requirement.