1. The behavioural and electrocortical (ECoG) power spectrum effects of clonidine, and yohimbine, an agonist and an antagonist at alpha 2-adrenoceptors, after their unilateral microinfusion into the rat locus coeruleus (LC) in young (50-70 days old) and old (13-15 months old) rats were studied. 2. Clonidine (0.09, 0.19, 0.28 and 0.56 nmol) microinfused into the LC of young rats induced dose-dependent behavioural and ECoG slow wave sleep (SWS) with a significant increase in total voltage power and power in the lower frequency bands. In contrast, yohimbine (1.3 and 2.6 nmol) infused into the LC of young rats produced ECoG desynchronization and a significant decrease in total voltage power. 3. In contrast to young rats, clonidine (0.19 and 0.28 nmol) given into the LC did not affect behaviour and the ECoG power spectrum in old rats. However, after higher doses of clonidine (0.56 and 1.2 nmol) a small and short-lasting period of behavioural and ECoG SWS was still evident. Similarly, in old rats yohimbine, at a dose (1.3 nmol) which was stimulative in young animals, did not significantly affect behaviour and ECoG power spectrum. Higher doses of yohimbine (2.6 and 5.2 nmol) were required to induce behavioural and ECoG changes similar to those observed with lower doses of yohimbine in young rats. 4. Chronic treatment with phosphatidylserine (30 mg kg-1, orally, daily for 21 and 30 days), was able gradually to restore in old rats, in comparison with a vehicle-treated group, the responsiveness of alpha 2-adrenoceptors to clonidine and yohimbine given into the LC.