The structure of the inhibitory domain of the inhibitor protein of the cAMP-dependent protein kinase has been assessed by circular dichroism studies of synthetic inhibitory peptides. Using the inhibitory peptide PKI(5-22)amide (Thr5-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn- Ala-Ile22) [Cheng, H.-C., Kemp, B. E., Pearson, R. B., Smith, A. J., Misconi, L., Van Patten, S. M., & Walsh, D. A. (1986) J. Biol. Chem. 261, 989-992] and shorter peptides of this sequence, it has been estimated that this parent peptide is composed of approximately 30% alpha-helix with the remainder being random coil with one beta-turn. The pseudosubstrate arginine cluster (Arg15-Arg19) is within the suggested region of random coil and beta-turn, representing one critical region of binding recognition by the protein kinase. The alpha-helix region proposed between Thr6 and Ile11 likewise contributes to the full biological potency and specificity of the inhibitor peptide and inhibitor protein. The removal of the two N-terminal threonines, for example, causes both a marked conformational change in the peptide and a diminishment by an order of magnitude of inhibitory activity. It is proposed that this alpha-helix region could serve one of several possibilities, including that it may provide a suitable constraint on the Tyr7 such that the hydroxyl is oriented in a position proximal to the pseudosubstrate domain, and/or may allow the optimal location of other protein kinase recognition signals. These data provide an initial description of some of the structural features of the inhibitor protein that could contribute to its high biological potency.