1. To examine the substrate specificity of renin a series of analogues of a noctapeptide previously shown to be an effective substrate (Skeggs, Lentz, Kahn & Hochstrasser, 1968) were synthesized. 2. These peptides were studied both with respect to their actions as competitive inhibitors and as substrates. 3. This resulted in the identification of two peptides which were potent competitive inhibitors but could not be cleaved by renin.