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Stress system is very important in the co-ordination of the generalised stress response, which takes place when a stressor of any kind exceeds a threshold. In our experiments we have analysed aspects of opiate involvement in the interactions among the emotional, motivational and cognitive processes, as well as aspects concerning the relations between physiological and behavioural mechanisms, both in normal conditions and under stress. We also analysed aspects of the interaction between stress system and endogenous opioid systems (e.o.s.). As emotional states interfere with perception and cognition, in our experiments we used suitable variants of the open-field test, which evaluates the exploratory drive and emotional reactivity. We used 30 Wistar male rats, 18 months old, weighing 350-400 g, divided into three groups, of 10 animals each. In order to assess the behaviour of the animals, we have analysed exploratory (cognitive) and non-exploratory (emotional) parameters. In order to evaluate the manner in which endogenous opioid systems are involved in these aspects, different groups of rats were treated with an opiate agonist (codeine syrup, 0.117, 5 ml/kg to approximately 0.5 mg/kg morphine as active compound, p.o.) and an antagonist (naloxone 0.4 mg/kg i.p.), before testing in open-field. To distinguish between the effect of habituation and the effect of the opiate agonist and antagonist, the placebo treatment and the test/retest method were used. At these doses, the opiate agonist has stimulated the exploratory behaviour, while the antagonist has inhibited the cognitive behaviour and enhanced the emotional response. Our experimental data together with data from literature allowed us to present an interactive model that points out a general manner of interaction between emotions and cognition, in relation to the interactions between physiological and behavioural mechanisms.