The activity of 2 types of Ca2+ channels (38 and 14 pS in 137 mM Ba2+) in the plasma membrane of the crayfish tonic flexor muscle is modulated by the peptide proctolin. This peptide serves as a cotransmitter in 3 of the 5 excitatory tonic flexor motoneurons and greatly enhances tension after depolarization by the conventional neurotransmitter. Proctolin alone has no effect on these channels, but renders them capable of sustained activity following depolarization. After depolarization induces activity, 5 x 10(-9) M proctolin increases the open probability of the larger channel up to 50-fold due to a marked decrease in the mean channel closed time. There is also at least a 4- fold increase in the percentage of patches with active channels for the large channel and a 2-fold increase for the small channel. Proctolin modulation appears to occur via an intracellular messenger, possibly cAMP. The peptide's effect on channel activity is dose dependent in a manner that parallels its effect on tension. These results indicate that the activation of these channels and the resulting influx of Ca2+ into the muscle fiber play a role in the potentiation of tension in this muscle.