Research has demonstrated that the majority of antihistamines (H1 antagonists) have sedative effects and can impair psychomotor performance; however, it is claimed that astemizole (hismanal) does not possess central nervous system side effects. A two-factor, repeated measures, double-blind design was used to compare the effects of three treatments (two antihistamines and one placebo) on cognitive information processing, mood, selected physiological measures, subjective feelings of drowsiness, and subjective performance ratings in 28 healthy men. Evaluations were given at 1,3,5,7,9,11,13, and 15 hours post ingestion. Time-of-day effects were evident in following directions, unstable tracking, code substitution, serial addition/subtraction, logical reasoning, manikin, and pattern comparison tasks. A general trend of improved scores through the day was observed and a temporal pattern of a low performance was suggested in the afternoon (2:00 pm and 4:00 pm). Temporal effects were noted for physiological measures. Benadryl produced performance decrements at one hour post ingestion on the following directions task, at one and a half hours on the unstable tracking task, and at three hours on the serial addition/subtraction task. No decrements in performance were found post ingestion of hismanal and, in fact, the hismanal group performed the serial addition/subtraction task more quickly than either the placebo or benadryl groups at five hours post ingestion. At three and a half hours post ingestion, the performance of the benadryl group remained poorer than the hismanal group on unstable tracking, but was not different from the placebo group. A higher level of tension, greater fatigue, and lower level of activity was experienced post benadryl. Lower vigor-activity and higher confusion-bewilderment post hismanal and benadryl were noted one hour post ingestion; however, confusion was lower and activity was higher for hismanal than benadryl. Low vigor-activity, high confusion, increased sleepiness, and low perceived performance post benadryl persisted for three hours, while fatigue-inertia persisted for seven hours. Subjects were able to determine receipt of a placebo versus an antihistamine following ingestion of either a placebo or benadryl. Results suggest that hismanal is superior to benadryl for avoidance of subjective effects and performance of information processing tasks. / Ph. D.