The effect of microelectrophoretic application of corticosterone (CS) on single neurones of raphe nuclei (RN) were investigated in rats under urethane anaesthesia. Ejecting currents generally ranged from 5 to 40 nA. CS produced an excitatory effect in 61% and no effect in 39% of the neurones. None of the 54 neurones studied was inhibited by CS. These quite homogeneous data support the hypothesis that RN are involved in the regulation of most of the nervous functions in which glucocorticoid hormones have been implicated.