Effect of PGE1 administered into anterior horn of lateral cerebral ventricle of unanesthetized monkey on changes of behavior and electrical activity of ipsilateral prefrontal cerebral cortex was studied. PGE1 in doses of about 50 μg/kg body weight caused tremor, disturbance of posture, catatonia and stupor for several minutes followed later by somnolence. The monkey also made occasional vocalizations during the phase of motor disturbance. Micturition and defecation responses also occurred. The electroencephalogram was desynchronized. Further, the rate of spontaneous discharge of impulses of some of the neurons of the prefrontal cortex was markedly decreased, up to about 33% of control. During the state of depression of spontaneous activity, the neuron could still be evoked to activity by electrical stimulation of dorsomedial thalamic nucleus. The duration of short latency negative wave evoked in the depth of the cortex by electrical stimulation of dorsomedial thalamus was not decreased but augmented under PGE1. This indicated that the decrease of spontaneous activity of the neurons under PGE1 could not be due to a postsynaptic inhibition of the neurons but probably due to alterations caused somewhere in the presynaptic chain. Low doses (about 25 μg/kg body weight) of PGE1 have not caused any disturbances of movements or posture, but caused drowsiness and facilitated sleep behavior. The results are discussed on the basis of previous reports on effects of PGE1 on adrenergic synaptic transmission in nervious system.