Tetrodotoxin blocks a graded sensory response in the eye of Limulus.

The generalization that tetrodotoxin selectively blocks all-or none electrical activity of nerve and muscle but has negligible effect upon graded responses of sensory systeins does not appear to be valid for the Limulus eye. Tetrodotoxin reversibly blocks the graded transient component of this visual response, while the steady state component of the response is rela tively unaffected by the drug.