In two consecutive partial studies conducted under double-blind conditions the potential effects of the antihistamine 1-(4-fluorophenylmethyl)-N-[1-[2-(4-methoxypenyl)ethyl]-4-piperidinyl]- 1H-benzimidazole-2-amine (astemizole) on the ability to drive and the safe operation of machinery was tested in a total of 85 volunteers. The test was performed with psychometrical methods and included the activation level (= objective tiredness), the subjective feeling of tiredness as well as their impact on concentration and reactivity. It was shown, that after a single administration (first partial study) there was no difference in performance between astemizole and placebo, whereas under the influence of the reference substance ketotifen concentration and reactivity were significantly impaired. It should be mentioned that the volunteers of the reference group did not yet experience a subjective feeling of tiredness, while their performance had already fallen off. Also after seven days of treatment (second partial study) with astemizole or placebo the active group did not exhibit any decline in performance. The authors advise against the generalized reference to a "possibly impaired reactivity" in the case of antihistamines.