1. Changes in the admittance of rod outer segments produced by illumination with brief flashes were studied by two methods: one, in which maintained changes in real and imaginary parts of admittance were observed in the frequency range 15 c/s-60 kc/s; the other, in which the time course of change in absolute value of admittance (Delta|Y|) was observed at frequencies of 100 kc/s-1.0 Mc/s.2. The response to light absorbed by rhodopsin was resolved into components. One of these components was a transient increase in conductance which arose from a rapid degradation into heat of the light energy. Another component, prominent at high frequencies where the conductivity of the rod interior was accessible to measurement, was produced by the uptake of H(+) by visual pigment in its conversion from metarhodopsin I to metarhodopsin II, causing a change in ionization of buffer.3. Two other components, designated I and II, appeared as maintained changes of admittance involving the organized structure of the rod. Component I appeared as a frequency-independent increase in the real part of admittance (DeltaG), the amplitude of which varied in proportion to the conductivity of the medium, without specificity as to ion species. Component II appeared as a DeltaG which rose linearly with log frequency over the range 1-60 kc/s, while the imaginary part of admittance change (DeltaB) rose to a plateau which was maintained for more than a tenfold frequency range. This component was unaffected by variations in conductivity in the region of low conductivities.4. When rods were suspended in a solution containing 100 mM hydroxylamine, component II no longer appeared as a maintained admittance change while component I was unaffected. Examination of the time course of response showed component II to appear transiently, decaying over the course of 2 sec following a flash.5. Measurements of Delta|Y| for rods in solutions of widely different conductivities showed component II to have a more rapid time course of development than component I and to be only slightly delayed in its early part relative to the buffer component.6. The amplitude of component I varied with temperature to the extent of 4.1%/ degrees C (Q(10) of 1.5) over the range -2-25 degrees C. The amplitude of component II was nearly constant over the range 15-27 degrees C, but fell steeply at temperatures below 10 degrees C, the Q(10) at low temperatures being about 2.4. The effect of temperature on amplitude and time course of component II is consistent with its dependence on the formation and continued presence of metarhodopsin II. The failure of component I to decrease steeply at temperatures below 10 degrees C indicates a dependence on an earlier stage in the thermal conversion of rhodopsin photoproducts.7. With light flashes each bleaching less than 1% of the rhodopsin content of the rod, all components of response were proportional to the amount of rhodopsin bleached (which would be proportional to the light absorbed). For brighter flashes components I and II failed to increase in proportion to the amount of rhodopsin bleached, the deviation from proportionality being greater for component I than for component II. The failure of summation of response extended to successive responses separated by up to 5 min.8. It is suggested that component I arises from a non-selective increase in ionic permeability of the surface membrane of the rod or from a change in rod volume, while component II arises from a change in conduction along the surface membrane.