The Limulus heart is neurogenic. If the cardiac ganglion is removed, all spontaneous contractions of the heart are abolished. Application of the pentapeptide proctolin (greater than 1 microM) causes the deganglionated heart muscle to beat with a frequency and amplitude slightly greater than those of a normal heart with an intact cardiac ganglion. At a proctolin concentration of 1 microM, rhythmic beating requires 2-10 min to develop, and up to 1 h of continuous washing is required to reverse the effect. A contracture often precedes the rhythmic contractions. Proctolin-induced rhythmicity occurs in the presence of tetrodotoxin (TTX) and in Na+-free saline. These effects of proctolin are not mediated by residual portions of the cardiac ganglion. Contractions are inhibited by Ca2+-free EGTA saline, CoCl2, MnCl2, and CdCl2. Proctolin causes no significant long-term changes in the myocardial resting potential or apparent input resistance. However, proctolin causes rhythmic 10- to 20-mV spikes that precede each contraction of the myocardium. Production of these spikes appears to be the mechanism by which proctolin causes rhythmic contractions in normally quiescent deganglionated myocardium of Limulus.