Stimulation of the optic nerve evoked two positive waves (P1 and P2) with short latencies and a following large negative wave (N) in the suprachiasmatic nucleus (SCN) of hypothalamic slice preparations. The conduction velocities of the P1, P2 and N waves were assumed to be 2.9 m/s, 0.5 m/s and 0.4 m/s, respectively. The N wave disappeared in low Ca2+ medium and all responses disappeared when TTX was added to the medium at a concentration of 10(-6) M. The results indicate that the P1 and P2 waves were responses of the optic nerve in the SCN and the N wave was that of SCN neurons. Following optic nerve stimulation 15 out of 212 single SCN neurons were excited; 10 were excited then inhibited, 14 were excited followed by rhythmic oscillation, and 4 were inhibited. The results demonstrate neuronal projections from the optic nerve to the SCN, and suggest that the major role of the excitatory influence on SCN neurons is responsible for the elevation of neuronal activity in the SCN during the daytime.