It is assumed that central histamine H1 receptor blockade, and to a lesser extent alpha-1-antiadrenergic and antimuscarinergic properties, play an important part in the sedative effect of quetiapine (11, 12). It is estimated that almost 100 % of the H1 receptors and over 50 % of serotonin 5HT2a and dopamine D2 receptors are blocked by use of 50 mg quetiapine (12). In other words, there is evidence that quetiapine exerts an effect on several receptor systems even in low doses.