Six adult monkeys (Macaca fascicularis) were trained to switch on a lamp by pressing the lever, to hold the lever for not less than 1 s (the lamp being switched on), and get a portion of food. After reaching a learning criterion, the delay between the lever pressing and food reinforcement was increased to 2.5 s. The experimental procedure was repeated in three experimental sessions with 2-month between-session intervals. It was shown that the retraining process after the uneven change in the delay duration developed in three stages: (1) stage of emotional hyperreactivity that reflected a mismatch between the cation and absence of the expected result; (2) stage of intermediate stabilization, when the percent of efficient attempts was the same as under conditions of 1-s delay; (3) stage of purposive instrumental lever holding till the moment of reinforcement presentation.