The present study compared the effects of clonidine and temazepam on performance on a range of tasks aiming to assess the role of central noradrenergic mechanisms in cognitive function. Fifteen healthy volunteers (seven male, eight female), aged 18-25 years, took part in a five-period crossover study in which they received placebo, temazepam (15 and 30 mg) and clonidine (150 and 300 microg) by mouth in counterbalanced order in sessions at least 4 days apart. A test battery was administered before treatment and at 45, 90 and 135 min after the dose. Performance on most tests was significantly impaired in a dose-related fashion, and subjective sedation was recorded for both drugs. The greatest impairments with clonidine were on attention in the presence of distractors. Clonidine did not affect the formation of new long-term memories, in contrast to temazepam, but did impair measures of working memory. Subjective effects, especially feelings of drunkenness and abnormality, were particularly marked with clonidine. These results support the suggestion that central noradrenergic function may be involved in preventing distraction, but do not confirm other reports suggesting that some aspects of performance are improved with clonidine.