The minimum latency of the a-wave.

1. Intense flashes of light evoke early and late receptor potentials from albino rat eyes.2. When the temperature is lowered the amplitude of the early receptor potential (e.r.p.) is found to decrease much more than that of the a-wave. Below 5 degrees the a-wave can be seen without a preceding negative e.r.p.3. The a-wave, but not the e.r.p., is suppressed by an intense flash, so that subsequent stimuli only evoke e.r.p.s.4. The responses of such light-adapted eyes have been used to determine the contribution of the a-wave to the response of dark-adapted eyes.5. These calculated a-waves begin during the e.r.p. with an initial portion which is biphasic.6. The initial portion has a temperature dependence similar to that of the a-wave. It is not related to the photoreversal of rhodopsin caused by the stimulating flashes.7. The minimum latency of the a-wave, extrapolated to body temperature, is approximately 300 musec.8. The site and mode of generation of the a-wave is discussed.