The United States posted a budget deficit of $31.46 billion in October in the opening month of fiscal 1991, the Treasury Department reported Friday.

When the current fiscal year ends next Sept. 30, it is expected the nation will post a record deficit, following a shortfall of $220.39 billion in fiscal 1990, the second-worst in U.S. history.October's federal budget deficit follows a surplus of $20.84 billion in September and a deficit of $26.08 billion in October 1989, according to the department's regular budget statement.

Last month's outlays totaled $110.17 billion, while receipts totaled $78.71 billion, the government said.

The United States is expected to run a budget deficit of at least $176.33 billion during fiscal 1991, according to administration estimates, and a shortfall of $133.88 billion in fiscal 1992.