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1. Early receptor potentials (E.R.P.s) were recorded with internal electrodes in turtle cones by applying brief flashes from a xenon tube with a maximum photon density equivalent to 2-3 x 10(8) photons micronm-2 at the optimum wave-length. 2. The E.R.P. was separated from the late receptor potential (L.R.P.) by superposing in flash on a step of light which was strong enough to saturate the L.R.P. 3. In red-sensitive cones the E.R.P. consisted of a brief depolarizing phase (R1) followed by a hyperpolarizing phase (R2) of maximum amplitude 10 mV and duration 30-40 msec. R1 was small or absent in green-sensitive cones. 4. With flashes of increasing intensity the E.R.P. approached its maximum exponentially with an exponential constant Q of about 10(8) photons micronm-2 which is of the same order as the reciprocal of the photosensitivity of porphyropsin; the implication of this result, which is considered in the theoretical section, is the the E.R.P. is proportional to the number of photoisomerizations. 5. When tested with a constant xenon flash at varying times after the beginning of a bleaching light the E.R.P. declined exponentially with a similar value of Q. 6. After prolonged bleaches the E.R.P. recovered with a time constant of about 100 sec but much quicker recoveries were observed after relatively brief bleaches. 7. The form and size of the E.R.P. are consistent with the accepted view that it arises from a redistribution of charge in the cone pigment molecule. 8. The effect of a single photoisomerization in an isolated cone was estimated as about 10(-10) V or one electronic charge through about 10% of the membrane.

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