1. Pericardial organs of Libima emargmata and L. dubia were isolated, the nerves from the thoracic ganglion were stimulated electrically, and propagated electrical activity was recorded. Fluid which bathed the neurohaemal organ was assayed for cardio-excitor effects on the isolated crab heart, when necessary, made tachyphylactic to drugs.

2. Neither 10-6M 5-hydroxytryptamine (5-HT), 10-5M dopamine (DA), nor 10-5M UML applied directly to the pericardial organ evoke release, nor do DA or UML alter release in response to stimulation.

3. The rate of cardio-excitor release is normal in pericardial organs taken from reserpinized animals. Histochemical examination of the pericardial organs by the formaldehyde-induced fluorescence method confirmed depletion of the monoamines.

4. These experiments, taken together, provide strong evidence against a role of monoamines in the release of cardio-excitor hormone from this neurosecretory structure.

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