S-potential and dark adaptation in fish.

1. The change in d.c. level of the S-space was investigated during dark adaptation to see if there is any correlation between the d.c. level and log sensitivity.2. Except for a few minutes after strong adaptation the d.c. level of the S-space was found unchanged throughout dark adaptation.3. The maximal excursion of the potential (the ceiling) also remained unchanged during the light and dark adaptation.4. Changes in the sensitivity in dark adaptation were found formally equivalent simply to a change in the unit of light intensity.5. Thus adaptations to bleaching or to the equivalent background are entirely different in the effect on S-potentials.