Evidence for a neuroactive component in the toxic extract from Gonyaulax monilata.

1. A toxic extract has been isolated from the red-tide dinoflagellate Gonyaulax monilata by a phenol-water extraction. 2. The compound action potential in the desheathed frog sciatic nerve is irreversibly blocked by topical application of the extract in concentrations as low as 40 ppm (w/v) in frog Ringer solution. 3. The toxin liberated by this algae has the potential for causing the neurological symptoms exhibited by animals following the administration of the toxic extract.