1. Bipolar surface electrodes were used to record electrical potentials from the brain of the goldfish, Carassius auratus L. Characteristic patterns of spontaneous electrical activity were described for telencephalon, mesencephalon, cerebellum and medulla oblongata. Changes in these patterns under deepening urethane narcosis were noted.

2. Rapid repetitive flashing light caused a change in the normal pattern of the mesencephalon which resembled the mammalian arousal. High-frequency (18-24 cyc./sec.) low-amplitude activity replaced the characteristic low-frequency waves (7-14 cyc./sec.); the lower-frequency activity returned 1.5 min. after cessation of stimulation.

3. Responses to single light flashes were described from the mesencephalon and cerebellum. An ‘A complex’ from the mesencephalon consisted of two to four rapid diphasic spikes, with a latency of 30-40 msec., duration of 40-50 msec., and amplitude of about 100 µV. A ‘B complex’ consisted of a large negative wave, duration 90-110 msec. and amplitude 100-200 µV., followed by a small positive deflexion, and a slow negative after-potential. The cerebellar response had a latency of 60-80 msec., duration of 100-120 msec., amplitude of 80-125 µV. The nature of these two components was discussed.

Monocular blinding abolished the response to light in the contralateral half of the optic tectum and in the cerebellum. The amplitude of the low-frequency spon-taneous activity characteristic of the mesencephalon was reduced in the contralateral tectum.

4. Regional differences in the tectal response to single light flashes were described. Microelectrodes were used to record the photic response from deeper tectal layers.

After destruction of parts of the retina by electrocautery, a disappearance of the A complex and 40-50% reduction of the B complex was observed in restricted tectal areas. The distribution of the areas of changed response was shown to correspond to a coarse overlapping projection of retinal quadrants of the fish eye on to the contralateral optic tectum. A possible basis for the localization of this projection, in agreement with anatomical and electrophysiological findings, was described.

This investigation was supported in part by Research grants from the National Science Foundation, no. C 4029 (J. P. S.) and no. 37085 (I. J. W.), and The Netherlands Organization for Pure Research (Z. W. O.), no. 96-44-66 (J. P. S.).