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The effects of electrical stimulation of prefrontal cortex upon neurons of substantia nigra (pars compacta) in anesthetized rats were mostly inhibition without antidromic excitation. By studying nigral neurons in which the inhibition from caudate-putamen was antagonized by iontophoretic bicuculline, it was found in only half of them that the same drug also antagonized the inhibition from prefrontal cortex. By recording single unit discharges of neurons of the substantia nigra, pars compacta (SN), the effects of electrical stimulation of the prefrontal cortex (FC) were studied in rats. In this paper we will report that the major effect from FC was inhibitory as was the case with stimulation of the caudate-putamen (Cd). In addition, we will describe that though the inhibition of SN neurons induced from Cd was mostly antagonized by iontophoretically applied bicuculline (BCL), such was seen in only a half of the cases with the inhibition induced from FC. Male albino rats, weighing 170 to 300 g, were used. They were anesthetized with urethane (1.25-1.3 g/kg, i.p.) and fixed in a stereotaxic apparatus by KONIG and KLIPPEL'S method (1963). Thereafter, additional doses of the same anesthetic were given as required. Bipolar electrodes, each consisting of two insulated wires glued together, were used for stimulation of FC and Cd. The stimulus site in FC was at 1.5 mm lateral to the mid-sagittal line and 3-5 mm rostral to the bregma, and in Cd it was at A 8.5-9.0 and L 2.5, 3.2-3.5 mm deep from the cortical surface. Stimulus pulses were 10-40 V in strength and 0.1-1.0 msec in duration. Usually they were applied at a frequency of 0.8-1.0 Hz. Extracellular spike discharges of SN neurons were recorded with glass