The rats systemic administration of tolonidine in rats (0.05-5.0 mumol/Kg) produced behavioural and electrocortical slow-wave sleep lasting between 35 and 200 min depending on the dose. In addition, a dose-dependent fall in deep body temperature was observed. Similar effects were evoked by infusing tolonidine (5-80 nmol) into the third cerebral ventricle. Behavioural, ECoG and body temperature effects were prevented by previous intraventricular or systemic injection of phentolamine, an antagonist at alpha 1- and alpha 2-adrenoceptors, and of yohimbine, an antagonist at alpha 2-adrenoceptors, whereas prazosin, an alpha 1-adrenoceptor antagonist, was ineffective. In conclusion, the present experiments show that tolonidine possesses central effects similar to those evoked by clonidine and provide further evidence in favour of the idea that central alpha 2-adrenoceptors are involved in the control of slow-wave sleep and body temperature.