They are typically fixed. There are exceptions like TIPS, Series I, and FRNs which are more complicated. See Treasury direct for more details: https://www.trea

They are typically fixed. There are exceptions like TIPS, Series I, and FRNs which are more complicated. See Treasury direct for more details: https://www.treasurydirect.gov/indiv/products/products.htm https://www.treasurydirect.gov/indiv/products/products.htm But even for the fixed bills, notes and bonds, they are typically rolled over into new ones when they mature. So the government is still exposed to changing market conditions. But it's not something that happens for all of the debt at once.