The regulatory peptides Phk13 (301-327) and a modified form of Phk5 (342-367) from the gamma-subunit of glycogen phosphorylase kinase form binary and ternary complexes with both calmodulin and the related muscle protein troponin C. Neither peptide appears to affect to a major extent a fluorescent probe linked to Cys-27 of wheat germ calmodulin. Phk13, but not Phk5, significantly modifies the properties of a probe joined to Cys-98 of troponin C. A comparison by means of radiationless energy transfer of the average separations of Trp-16 of Phk5 from specific groups in the N- and C-terminal halves of calmodulin and troponin C indicate significant changes upon going from the 1:1 binary complex to the 1:1:1 ternary complex with Phk13. A comparison of the effects of addition of Phk13 to calmodulin, troponin C, and their binary complexes with Phk5 suggests that the conformation of Phk13 is similar in the binary and ternary complexes.