Several investigations have revealed substantial influences of pharmacological manipulation of central noradrenergic activity upon performance in cognitive tests sensitive to frontal lobe dysfunction. They suggest a significant role for the noradrenergic coeruleo-cortical projection in cognitive function but conflicting findings and the complex pharmacology of adrenoceptor agents make it difficult to be precise about underlying mechanisms. In order to clarify these we have compared the effects of an alpha1/alpha2-adrenoceptor agonist, clonidine, an alpha2-adrenoceptor antagonist, idazoxan, and these agents in combination. Three groups of healthy volunteers were used to investigate the effects of these noradrenergic manipulations upon performance of tasks from the CANTAB test battery known to be sensitive to frontal lobe dysfunction. Previously reported effects of clonidine upon sustained visual attention and upon session-to-session improvement were replicated. Furthermore, idazoxan inhibited the hypotensive effect of clonidine. Idazoxan had no overall effect on performance of any of the tests but did inhibit session-to-session improvement in performance of a planning task, attentional set shifting and sustained visual attention. Rather than leading to the anticipated mutual antagonism of effects, combining clonidine and idazoxan led to a wider and more striking range of cognitive impairments. These results are discussed alongside findings which support a role for imidazoline (I1) receptors in blood pressure control, where clonidine and idazoxan are antagonistic, and evidence of less potent antagonism at somato-dendritic alpha2-adrenoceptors in the locus coeruleus.