The Regional Transportation Authority says service may have to be cut on the CTA, Metra and Pace as soon as next month if the state does not pay up.
Illinois is supposed to pay $450 million a year toward the cost of mass transit in the Chicago area, but the RTA says the state is behind by nearly $400 million.
Transportation officials say they have borrowed all they can to cover the shortfall.
They have sent a letter to Governor Pat Quinn and state Comptroller Judy Baar Topinka asking for some of the money owed.
Despite the threat by the RTA, Metra officials say they can get through the year without problems, and the CTA says it is not planning service cuts.