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Formation of trace processes was studied by means of delayed reactions with a choice, in intact animals and in animals with an ablated dorsal part of the hippocampus, aged six weeks, three and twelve months. It has been found that with age the percentage of correct solutions increases in intact animals, as well as the duration of permissible maximal delay. Lesion of the dorsal part of the hippocampus in six-week and three-month old puppies leads to a complete disturbance of the capacity to retain short traces. In one-year old puppies the operation results in a considerable drop in the level of correct solutions with short delays (5 to 20 sec). The longer the delay, the fewer the number of adequate solutions. With long delays the number of adequate responses is at the level of random choice.

In 2-9 months dogs the influence was studied of ablation of the hippocampus dorsal area on formation and preservation of conditioned taste aversion (CTA), elaborated by combination of 30% sucrose solution with i.p. injection of 0.28 M LiCl solution. Lesion of the hippocampus dorsal area does not prevent acquisition after the first pairing of conditioned taste aversion in puppies and adult dogs. Heterogeneous influences are observed after hippocampus lesions on the process of CTA extinction in animals of different ages. Acquaintance with conditioned stimulus before CTA acquisition accelerates the process of its extinction in hippocampectomized indiviuals, but less than in animals with an intact hippocampus.

The formation of trace motor and secretory alimentary conditioned reflexes were studied in dogs after injury of dorsal part of hippocampus at different stages of ontogenesis, 38--42 days, 3, 5, and 13 months. All animals with lesioned dorsal hippocampus formed non-delayed motor and secretory reflexes. Trace motor reflexes were elaborated in four out of nine animals, and trace secretory ones--only in one animal. The experiment emphasized the importance of the degree of hippocampal injury for the maintenance of the studied functions. Morphological analysis revealed that degeneration of thalamic nuclei is identical in animals operated at different age. No signs of brain tissue regeneration were observed.

The conditioned taste aversion (CTA) in dogs of different age including suckling puppies, is elaborated in one combination of conditioned stimulus (taste of 30% sucrose solution) with discomfort, elicited by parenteral injection of 0.28 M of LiCl solution. CTA persists up to a week and a half in mother-fed puppies, whose long-term memory for exteroceptive signals is not yet formed. In elaboration of CTA participation of not only intero- but also exteroceptive signals has been revealed even at early stages of ontogenesis. The longest preservation of CTA is common to animals of exaltative period of development. However, here two subperiods may be singled out characterised by different ability to CTA extinction depending on presence or absence of previous acquaintance with conditioned stimulus (2-2.5 and 3-4 months). The time of CTA extinction in suckling puppies does not depend on the degree of previous acquaintance with conditioned stimulus.