This paper shows that the spread between the 3-month Treasury bill and the federal funds rate has significant predictive power for the future change in the federal funds rate during the volatile nonborrowed reserves operating regime, but it has less and no predictive power during the borrowed reserves regime and the federal funds targeting regime, respectively. These findings suggest that Treasury bill rates forecast future federal funds rates most accurately when the Federal Reserve follows a well-defined rule that does not smooth the impact of shocks on the federal funds rate.