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1. The field potential profile resulting from electrical stimulation of the optic nerve-head was recorded with micro-electrodes inserted radially into the pigeon optic tectum.

2. The first response, in the superficial tectum, was a graded negative wave, the N-wave. Its latency was 2·3 msec, S.D. 0·22 msec. The wave form underwent an abrupt polarity reversal in the R-zone, to yield a graded positive wave, the P-wave, in deeper laminae. The P-wave had a latency of 2·0 msec, S.D. 0·16 msec.

3. Both N-wave and P-wave showed a graded increase in amplitude as stimulating current was increased. On any given track this increase occurred over a similar range of stimulating current.

4. In many tracks the rise-time and duration of the N-wave were independent of amplitude.

5. The rise-time of the P-wave was longer, and its duration was shorter, than the rise-time and duration of the N-wave. This effect was shown to be due to two successive sinks occurring superficially to the P-zone.

6. Depth profiles also showed these two sinks, separately located in the N-zone and R-zone.

7. To paired stimuli the N-wave showed refractoriness at intervals of 2-4·5 msec, facilitation at intervals up to 25 msec, and subsequent depression. It showed low-frequency depression at 10 and 20/sec.

8. The R-zone was localized by a lesion technique to sub-layers d to g of the stratum griseum et fibrosum superficiale. Thus the tissue negativity was closely co-extensive with the anatomical distribution of optic nerve terminals.

9. It is suggested that the N-wave is the sink of excitatory synaptic potentials produced in the superficial tectum, and predictions relating unitary firing to the field potential profile are made.