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Department of Visual Science, Institute of Ophthalmology, Judd Street, London WC1H 9QS

1. The electroretinogram (e.r.g.) of the isolated rat retina has been investigated by recording potential differences developed between two micropipettes.

2. In the uniformly illuminated receptor layer, voltage gradients at 90° to the long axes of the receptors are negligible in comparison with the radial voltage gradients.

3. When all transsynaptic neural activity has been abolished, the photoresponse recorded across the receptor layer is very different from the photoresponse recorded across the inner retinal layer.

4. The photoresponse developed across the inner retinal layers, slow P III, develops slowly and the peak voltage is approximately proportional to log. flash energy.

5. The photovoltage across the receptor layer rises rapidly to its peak, before a significant fraction of slow P III has developed.

6. The faster photovoltage (receptor potential) increases with flash intensity according to the hyperbolic function characteristic of photo-receptors.

7. The faster photovoltage can be split into two components. Between the tips of the outer limbs and the bases of the inner limbs, it has a simple wave form. In the region between the bases of the inner limbs and the receptor synapses, there is an additional peak (nose) to the photovoltage.

8. In the scleral portion of the receptor layer, the photovoltage approximately equals the dark voltage. In the remaining, vitreal portion of the receptor layer the photovoltage exceeds the dark voltage.

9. Photocurrent divergence has been measured and the results indicate that the source of photocurrent extends further vitreally than the base of the outer limb.

10. The results suggest that the photoresponse generated in the outer limb is modified by an active process which occurs in portions of the rods which are nearer the synapse.