It is known that the individual features of an animal are manifested, in particular, in the situation of conflict between the character (value) of reinforcement and the probability of its satisfaction [2]; it had been demonstrated previously that the forecasting of the probability of reinforcement is linked to the functioning of the anterior divisions of the neocortex and hippocampus, and that the functional basis of the choice of the character of reinforcement is the activity of the hypothalamus and the amygdala [I]. As a continuation of these studies we investigated the influence of the simultaneous change in the functional state of the two structures responsible for the assessment of the character or the probability of reinforcement. By contrast with the previous studies, we did not use the extirpation method, but rather the transplantation of embryonal tissue of the investigated brain structures. The experiments were carried out on 3- to 3.5-month-old mongrel male rats. The chamber in which the investigations were carried out was a rectangular box with two drink dispensers on the front panel. Access to these was restricted by Plexiglas corridors in such a way that, once having chosen a direction of movement, the rat could obtain reinforcement either on the right or the left. The turning on of a 60 watt lamp after the animal returned to the starting place, which was located in the rear portion of the experimental chamber, served as the conditional signal. The number of effective approaches, the number of intersignal reactions, i.. e., the number of shifts from drink dispenser to drink dispenser during the 1-2 min between the presentations of the conditional stirnuh, as well as the state of motoric inhibition of the animal, were recorded. This last parameter was arbitrarily defined by us as the case in which a rat did not exit from an arm of the maze to the starting place 1-2 rain following the presentation of the conditional signal. The animal received 10 presentations of the conditional signal in the course of each experiment. A 0.2 ml portion of water served as the reinforcement. The transplantation was accomplished in the following manner: one of four pairs of embryonal tissue of the frontal cortex and hippocampus (n = 20), arnygdala and hypothalamus (n = 19), frontal cortex and hippothalamus (n = 14), and hippocampus and amygdala (n = 17) was introduced into the parietal region of the cortex of one of the hemispheres through medial or lateral trephined openings. The coordinates of the trephined openings were as follows, in ram: of the medial, AP 1.5, L - 1.0-1.5; of the lateral, AP - 1.5, L - 3.0-3.5. The depth of the implantation was 2.0-2.5 ram. Transplants measuring 0.1-0.3 mm 3 were introduced in a volume of 0.1 mm [sic] of physiological solution. Physiological solution in the same volume as in the case of the experimental rats was introduced into the pariel~ region of the cortex of one of the hemispheres in 20 sham-operated (control) animals. The choice of site of implantation in the cortex was based on the following considerations: 1) the damage to the associative parietal cortex did not induce substantial disturbances in the behavioral reactions we were studying in the periods of testing investigated by us; 2) the creation of identic~ conditions for the transplantation of different pairs of structures; and 3) mechanical injury to the same structures of the recipient's brain as the transplants was excluded in the process of implantation into the cortex. The data obtained were evaluated statistically using the Student test. It should be stated that, in planning the experiments in a T-maze, we wished to determine how the transplantation of the different pairs of brain structures effect the behavior of rats in the presence of reinforcement of different characters and different probabilities. However, as early as at the first stage, when the rats were trained to drink water from the right and left drink dispensers, we ran up against the fact that we were observing lateralization of the choice of drink dispenser as a function of which of the hemispheres the embryonal nerve tissue was transplanted from.