The purpose of this study was to detect the behavioural effect of drug-induced changes in the neostriatal dopaminergic activity upon the degree of intrinsic (self-generated) and extrinsic (externally produced) constraints on the selection of behavioural patterns in rats. Both systemic and neostriatal injections of extremely low doses of apomorphine and haloperidol were used to change the neostriatal dopaminergic activity. Behavioural changes were observed in (a) an open-field test, (b) a so-called 'swimming without escape' test, (c) a so-called 'swimming with escape' test, and (d) a test to detect deficiencies in sensory, motor and sensorimotor capacities required to perform both swimming tests. Evidence is found that the neostriatum, especially the neostriatal, dopaminergic activity determines the animal's ability to select the best strategy in a stressful situation by modifying the process of switching strategies under pressure of factors intrinsic to the organism: neither sensory neglect nor inability to initiate voluntary movements underlay the observed phenomena. It is suggested that the neostriatum determines the individual flexibility to cope with available sensory information.