The alpha 2 adrenoceptor has recently been implicated in working memory (WM), a function dependent on the integrity of the prefrontal cortex. Using a double-blind, placebo-controlled design, the present investigation examines the effects of two doses (1.5 micrograms/kg and 2.5 micrograms/kg) of the mixed alpha 1/alpha 2 adrenoceptor agonist clonidine (CLO) on performance of various computerised tests of WM and planning in healthy, young volunteers. These are compared to the effects produced by two doses (5 mg and 10 mg) of diazepam (DZP) on largely the same set of neuropsychological tests in a comparable set of subjects. Administration of CLO resulted in impulsivity of responding in a planning task, as well as differential dose-dependent effects on two analogous tests of spatial and visual WM. The nature of these effects were suggestive of mnemonic, rather than executive, dysfunction. Conversely, DZP produced specific deficits on tests of spatial WM and planning very similar to those seen following lesions to the frontal lobes. Therefore, these two sedative drugs produce doubly dissociable, dose-dependent effects on different aspects of cognitive function.