The Limulus heart consists of a relatively simple neural network, the cardiac ganglion, and its target tissue, cardiac muscle. The large size and exceptional in vitro viability of this system has made it relatively easy to extract, purify, and identify endogenous compounds which alter cardiac function. These agents included peptides, such as protolin and Limulus chromatophorotropic factor, and amines such as dopamine, epinephrine, norepinephrine, octopamine, and serotonin. The accessibility and simple organization of the cardiac ganglion has also permitted clear identification of the sites of action of these amines and peptides. The Limulus heart is thus a very favorable system for studying peptide and amine neurohormones at the network, cellular and molecular levels.