In experiments on spontaneously active neurones of isolated CNS of Lymnaea stagnalis the influence was studied of automatic contingent intracellular electrostimulation on the character of spike discharge of these cells. In every tested neurone in some series of stimulation only prolonged interimpulse intervals (greater than their mean background value) were selectively reinforced in other series--only short ones. It is shown that at such contingent reinforcement both of long and short intervals 35% of neurones can directly change the frequency and the pattern of their spontaneous impulse activity to "minimize" ("maximize") the number of autoreinforcements in both regimes of stimulation, what may be considered as a cellular analog of the instrumental reflex. Control rhythmical stimulation of the same cells elicited no significant changes in the character of spontaneous activity. It is suggested that such contingent intracellular electrostimulation is biologically significant for the cell, as it elicits directed reconstruction of the initial discharge pattern, leading to minimization or maximization of the frequency of autoreinforcements. A possible contribution is discussed of such endoneuronal plastic reconstructions in the activity of the neuronal network participating in the performance and regulation of conditioned acts.