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1. This paper describes synaptic actions upon the isthmo-optic nucleus.

2. A synaptic field potential, the S-wave, can be recorded from the nucleus in response to electrical stimulation of the lateral tectum. It cannot follow repetitive stimulation at rates higher than 30/sec, is depressed for long periods following the A-wave, and shows frequency potentiation at 10/sec.

3. The majority of output cells in the nucleus are fired synaptically from the lateral tectum. The early discharge corresponds in timing to the S-wave, and represents monosynaptic activation via the tecto-isthmo-optic tract. Later firing is produced by more complex routes. The early firing shows frequency potentiation at 10/sec.

4. An estimate is given of the time at which a centrifugal volley enters the retina after electrical stimulation of the lateral tectum.

5. One tenth of the output cells in the nucleus are fired synaptically from the retina. Short latency firing occurs at 5-7 msec, and is often blocked by prior antidromic invasion. Cells are also fired synaptically at longer latency, at 80-110 msec.

6. Antidromic invasion can be blocked by conditioning stimuli applied to the retina and to the lateral tectum.

7. It is shown that even the earliest synaptic firing from the retina could involve the serial activation of the retinotectal and the tecto-isthmo-optic systems.