1. The action of noradrenaline on the membrane properties of rat periaqueductal grey (PAG) neurones was examined using intracellular recordings in brain slices maintained in vitro. Morphological properties and the anatomical location of neurones were characterized by use of intracellular staining within biocytin. 2. Noradrenaline (0.3-100 microM) depolarized 66% (81/123) and hyperpolarized 30% (37/123) of neurones. The alpha 1- and alpha 2-adrenoceptor agonists phenylephrine and UK 14304 produced depolarizations and hyperpolarizations in all PAG neurones tested, respectively. Neurones depolarized by noradrenaline were more responsive to phenylephrine, whereas neurones hyperpolarized by noradrenaline were more responsive to UK 14304. 3. The UK 14304-induced hyperpolarizations reversed polarity at -108 +/- 2 mV (n = 11). The reversal potential increased when the extracellular potassium concentration was raised (slope = 57.8 mV/log[K+]o mM) in a manner similar to that predicted for potassium conductance. 4. The phenylephrine-induced depolarizations did not reverse polarity at negative potentials (n = 25), or did so at potentials (-119 +/- 2 mV, n = 13) more negative than the UK 14304-induced hyperpolarizations. Superfusion with low calcium (0.1 mM), high magnesium (10 mM) and either cobalt (2-4 mM), or cadmium (100 microM) usually reduced the response to phenylephrine and produced reversals near that predicted for potassium conductance. 5. The majority of the ventrolateral PAG neurones were depolarized by noradrenaline (85%, 62/73). In contrast, almost equal proportions of the lateral PAG neurones were hyperpolarized (54%, 20/37) and depolarized (46%, n = 17/37) by noradrenaline. PAG neurones depolarized or hyperpolarized by noradrenaline could not be differentiated on morphological grounds. 6. These results suggest that the net effect of noradrenaline on lateral and ventrolateral PAG neurones is to bias activity in favour of a ventrolateral PAG-mediated response pattern, which includes quiescence, hyporeactivity, hypotension and bradycardia.