The suprachiasmatic nucleus (SCN) of female rats was surveyed with microelectrodes under urethane anaesthesia. In rats with bilateral transection of the optic tracts, repetitive three pulses of 100 Hz applied to the contralateral optic nerve excited 8 and inhibited 11 other of the 86 SCN units examined. Transection of the optic tract did not significantly influence frequency of occurrence of the SCN units that were excited or inhibited by stimulation of the optic nerve. Certain SCN units responded to both of contralateral and ipsilateral stimulations of the optic nerve, indicating that bilateral visual inputs converge on the same single SCN neurones. Oscillatory responses with a period of 100--200 msec were occasionally produced by stimulation of the optic nerve. Flash stimuli with relatively weak intensity, even insufficient for producing wavelets in electroretinograms, produced an excitation and inhibition in SCN units. The mean firing rates were significantly altered by either electrical or flash stimuli repeated 500 times at 0.97 Hz in those units which showed no transitory response. Some of the SCN neurones receiving visual inputs were identified to be the tuberoinfundibular neurone and some other SCN neurones were found to receive converging inputs both from the optic nerve and from the axon collaterals of tuberoinfundibular neurones.