1. Doses of clonidine 0.15 mg or guanfacine 1.0 mg, respectively, and 2 h later additional doses of clonidine 0.3 mg or guanfacine 2.0 mg, respectively, were given to 24 healthy students. 2. Blood pressure was reduced by the same amount by both drugs. 3. Plasma noradrenaline concentrations decreased with both drugs, but the reduction was significantly greater following the administration of clonidine. 4. Mental activity in the EEG was less suppressed in the guanfacine group than in the clonidine group. The differences were statistically significant. 5. Self-estimations for well-being and mood showed only small changes due to guanfacine but significant changes due to clonidine. 6. The decrease of information processing and the increase in reaction time, measured by performance in different psychometric tests, were significantly more pronounced after clonidine treatment than guanfacine. 7. A dose-response relationship could only be observed in vigilosomnograms, in the tests of self-estimation related to well-being and mood and in the decrease in plasma noradrenaline in the clonidine group. 8. It was concluded that guanfacine had a lesser CNS depressant action than clonidine, when administered in equipotent hypotensive doses.