It has recently been shown that for a single act of learning to take place two nuclear formations must be present --the amygdaloid complex (AC) and the central gray matter (CGM) of the midbrain [i, 6, 8], which are probably united into single morphological and functional systems with definite connections and structures. The search for these structures and connections is very important for the complete elucidation of the whole system, with its direct role in the mechanisms of memory.One possible channel of communication is the subthalamus, which is connected with AC through the medial forebrain bundle [9] and with CGM by the bundle of Schutz [5].For the reasons given above we were interested in the role of the subthalamus and, in particular, of the zona incerta and Forel's fields, in the formation of the conditioned passive avoidance reaction.The experiments were carried out on 65 male Wistar rats weighing 170-200 g. A conditioned avoidance reaction (CAR) was formed in the animals by the method fully described previously [2]. The apparatus consisted of two compartments: a "safe" illuminated and a "punishable" dark compartment. The latent period of passage (LPP) from the light into the dark compartment was recorded during observations on each animal lasting 180 sec. On the 4th day of familiarization, after a routine passage from the illuminated into the preferred dark compartment, an electric current of 0.75 mA was applied to the metal floor, and against this background the CAR was formed as usual in the overwhelming majority of intact rats, or a current of 1.5 mA was applied, which led to the formation of a conditioned reflex in 100% of animals. The formation and preservation of the conditioned reaction were verified after 24 and 48 h. During the choice of coordinates for bilateral electrolytic destruction of z. incerta and Forel's fields, the stereotaxlc atlas of K~nig and Klippel [7] was used (z. incerta --A = 4.8 mm; L = 1.0 mm; H = 7.5 mm; Forel's fields --A = 3.2 mm; L = 1.2 mm; H = 8.0 mm). For destruction of z. incerta and Forel's fields, a direct current of 2 mA was applied for 20 sec.Conditioned reflex formation began one week after the operation. If a CAR was absent, the rats were given a "reminder" e!ectriccurrent in a situation which differed from that in which learning took place [3]. Testing was used 24 h after the "reminder."During conditioned reflex formation the animals of the control group and those undergoing the operation responded equally to the electric shock. No differences were found in the intensity of the motor or autonomic manifestation of the responses.In the first experiment a current of 0.75 mAwas used as the unconditioned reinforcement. As Table 1 shows, the mean values of LPP before training, in the animals undergoing the operation, were indistinguishable from the mean value of LPP in the control. The differences appeared clearly after training. Whereas in animals of the control group LPP increased sharply when tested after 24 and 48 h, rats with destruction of z. incerta (Pig. i)...