Calmodulin-binding peptides, which had previously been isolated from a pepsin digest of alpha-CN, were synthesized and then examined for their inhibitory effects on the activation of cyclic nucleotide phosphodiesterase that was induced by calmodulin. The concentrations of the synthetic peptides corresponding to 164-179, LKKISQRYQKFALPQY; 183-206, VYQHQKAMKPWIQPKTKVIPYVRY; and 183-207, VYQHQKAMKPWIQPKTKVIPYVRYL, of alpha s2-CN that gave half-maximal inhibition were 65, 7.0, and 2.6 microM, respectively. These inhibitory effects were reversed by increasing the amount of calmodulin. Fragments and analogs were prepared to study the interactions of the peptides with calmodulin in more detail. The results indicated that modification of the carboxyl terminus enhanced the affinities of the three peptides for calmodulin, and a region involved in the inhibition by alpha s2-CN (f183-207) was located at the carboxyl terminus 191-207. Two predicted calmodulin-binding sequences, 164-179 and 191-207 of alpha s2-CN, despite rather divergent primary structures, shared the structural motif common to the calmodulin-binding domains of the target proteins in the previously proposed complex model.