We have studied the effects of three antihistamines on task performance in two separate experiments. Healthy subjects were tested at weekly intervals in a double-blind, Latin square design. In Experiment I the subjects were treated orally with loratadine 10 mg, clemastine 1 mg, terfenadine 60 mg, or placebo. In Experiment II 5 mg diazepam was given orally with each of the four treatments used in Experiment I. In both experiments subjects' performance was evaluated in reaction time and tracking tasks after treatment. In both experiments, the tracking task initially was performed alone and then simultaneously with a continuous memory task; the subject also graded their mental status on visual analogue rating scales. In both experiments task performance was not generally impaired after treatment with loratadine or terfenadine. The concomitant administration of diazepam in Experiment II appeared not to affect subjects' performance. However, clemastine caused a decay in subjects' performance in both Experiments I and II, but only on the tracking task. At the conclusion of both experiments, sleepiness was reported by more subjects when treated with clemastine than when treated with loratadine, terfenadine, or placebo.