The dynamics of the changes in the number of instrumental motor reactions and in the frequency of cardiac contractions and of the theta rhythm of the hippocampus during the replacement of constant reinforcement by various schedules of probabilistic reinforcement of an alimentary conditional stimulus were investigated in three dogs. At the 70 and 50% level of reinforcement, the instrumental reflex appeared in response to all of the presentations of the signal stimulus. The greatest decrease in the number of respondent reactions was observed at 5% probability. The pulse rate both in the intersignal period and during the receipt of an empty feeder depended on the probability of reinforcement employed and on the individual characteristics of the animals. The frequency of the theta rhythm in all of the dogs was highest at the 30% level; the lowest values were observed at the 5% level of reinforcement. The data obtained again attest to the complex genesis of emotional excitation, the magnitude of which depends both on the informational (indeterminacy) and on the motivational (the significance of the reinforcement) factors.