1. The pericardial organs and anterior ramifications (both neurohaemal structures) of six species of crabs have been examined as whole mounts by the histochemical method for monoamines based on formaldehyde-induced fluorescence.
2. A small number of specifically fluorescing axons (not more than six green and six yellow) innervate the pericardial organ; one of the green-fluorescent and one yellow-fluorescent axon branches and also innervates the anterior ramification.
3. All of the fluorescing axons enter via segmental nerves 1, 2 and 3 from the ventral ganglion.
4. One large, brilliant green-fluorescing axon, and the small green-fluorescing axon which branches to the AR, have been traced in Carcinus to cell bodies in the circumoesophageal connective ganglion. These cells may give rise to the entire population of green-fluorescing axons and terminals in the neurohaemal organs.
5. Each axon, throughout its course in the pericardial organ, supplies a dense array of varicosities (blebs) at surfaces which are directly exposed to the haemolymph. The anterior ramifications are also supplied with blebs.
6. Lack of fluorescence in controls not exposed to paraformaldehyde, reversible quenching of fluorescence by treatment with sodium borohydride, and depletion of the fluorescence by reserpinization of the crabs, all confirm that the fluorescence is specific and represents the intracellular localization of monoamines.
7. With the aid of data available elsewhere we conclude that there are distributed, in parallel with peptide-secreting axons and terminals in the pericardial organs and anterior ramifications, a group of dopamine-containing and a group of 5-hydroxytryptamine-containing axons and terminals.