The present study describes the behaviour effects of intracerebral injections of the noradrenergic (NE) agonist oxymetazoline and the NE antagonist phentolamine into the "limbic" part of the caudate nucleus of cats primed 24 hr earlier and/or treated acutely with morphine (5 mg/kg, IP). Drug-induced changes in the morphine-specific behaviour served as dependent variables. Experiments were performed during two different periods of the year, each of them marked by a characteristic sensitivity of alpha-like NE receptors to NE agents, viz. the so-called NE "antagonist" period during which the NE receptors were sensitive to the NE antagonist phentolamine and the so-called NE "agonist" period during which the NE receptors were sensitive to NE and the NE agonist oxymetazoline. The present study demonstrates that morphine reversed the initial sensitivity to oxymetazoline respectively insensitivity to phentolamine in animals tested in the NE "agonist" period. In animals tested in the NE "antagonist" period morphine did not reverse the initial insensitivity to oxymetazoline resp. sensitivity to phentolamine. Furthermore, evidence is provided that the initial sensitivity to NE agents did not conspicuously determine the animal's response to the acute administration of morphine. The data are discussed in view of the concept that the firing rate of NE fibres determines the actual sensitivity of presynaptic and postsynaptic NE receptors to NE agonists and antagonists.