We quantify the impact of explicit FOMC policy rate guidance used as an unconventional monetary policy tool at the zero lower bound of the policy rate on market interest rates. We study the impact on short- to medium-term interest rates implied by Eurodollar interest rate futures contracts, and on near- to long-term interest rates implied by US Treasury securities. We find that explicit policy rate guidance announcements significantly reduced interest rates implied by Eurodollar futures at horizons of 1 to 5 years ahead, with the largest effect at the intermediate horizon of 3 years. We also find that they significantly reduced forward interest rates implied by US Treasuries at horizons of 1 to 7 years ahead, with the largest effect at the intermediate horizons of 4 and 5 years. Moreover, we find that explicit FOMC policy rate guidance led to a significant reduction in the term spread, ie to a flattening of the yield curve, both for the Eurodollar futures curve and the US Treasury yield curve.