Stable intracellular recordings were obtained from 22 suprachiasmatic neurones in isolated brain slices. These cells were characterized by resting potentials of about -60 mV, high input resistances, relatively short time constants and action potentials of short duration. The action potentials were preceded by a slow depolarization and followed by a relatively brief afterhyperpolarization and long-lasting increase in membrane conductance. Current-voltage relations were usually linear between 0 and 80 mV negative to the resting potential. Postsynaptic potentials were evoked in these cells by electrical stimulation of the optic chiasm or contralateral suprachiasmatic nucleus. Both excitatory postsynaptic potentials, which evoked action potentials, and inhibitory postsynaptic potentials were recorded. Synaptic potentials were associated with an increase in membrane conductance. Action potentials evoked by synaptic activation were sometimes followed by up to three small, fast potentials. Small fast potentials were not seen to occur spontaneously, or to follow spontaneous, or current-evoked spikes, nor were they evoked by synaptic potentials that failed to evoke action potentials. The suprachiasmatic nucleus is essential for the generation of normal biological rhythms in mammals. The input it receives from the optic nerve is thought to be important in this role. It is hoped that these preliminary intracellular studies will form a basis for further work on the inherent properties of suprachiasmatic neurones and their responses to visual input.