The hippocampal theta rhythm was studied during an adap­tive behaviour elaboration (a receiving of drinking reward in response on a specific audio stimulus) in freely moving water deprived rats. It was revealed that a right behavioral response (a drinking tank visit) in conditions of discrimination of two sound stimuli emerged only in case of the theta rhythm fre­quency increase in response on the positive conditional stimu­lus. Although this increase occurred only in response to posi­tive signal it was also linked with the differentiation process -the theta rhythm frequency increase became significant when animals started to recognize positive and negative conditional stimuli. The detailed analysis revealed the detected theta rhythm frequency increase consisted of two waves. One of them was connected with the positive conditional stimulus presentation moment. The other one was connected with the time of ap­pearance and duration of movement to drinking tank after positive conditional stimulus presentation (right behavioral answers) as well as after negative conditional stimulus (wrong answers). At the same time the frequency was significantly higher during right behavioral answers comparing with wrong ones. It is concluded that the theta rhythm frequency increased during the sensory information analysis when it was needed to choose a signal leading to reinforcement and to exit from the state of the internal inhibition. On the other hand the fre­quency increased during the purposive movements.