1. Saline which had bathed an isolated crab pericardial organ was chromatographed on a column of Sephadex G-25. The fractions were divided and assayed for cardioexcitor activity and for ninhydrin colour following hydrolysis.
2. Fluid from pericardial organs which had been stimulated electrically to give a maximum propagated compound action potential showed both cardio-excitor activity and ninhydrin colour. These were always in the same fractions, which corresponded to the volume for elution of small peptides. Fluid from unstimulated preparations gave negative assays.
3. The cardio-excitor activity of fluid from stimulated pericardial organs and of distilled-water homogenates of pericardial organs behaved identically in Sephadex G-25 chromatography.
4. No evidence could be obtained of the cardio-excitor peptide binding to a protein.
5. This work provides evidence that neural stimulation of crab pericardial organs results in release to the bathing fluid of cardio-excitor hormone as free peptide.