OBJECTIVE To compare the central and peripheral H1 inhibitory effects of acute and sub-chronic doses of levocetirizine (L-CTZ), cetirizine (CTZ), loratadine (LOR) and promethazine (PRM) versus placebo, using a battery of psychomotor and cognitive tests together with measures of the weal and flare reaction. PRM was included in the study as a positive internal control to validate the sensitivity of the psychometric test battery to the CNS effects of the various treatments. METHODS Twenty healthy volunteers (18-50 years) received L-CTZ 5mg, CTZ 10 mg, LOR 10 mg, PRM 30 mg and placebo once daily for four days in a five-way, double-blind, crossover study. For each treatment condition, subjects were assessed using a psychometric test system and a pinprick weal and flare response to 100 mg/ml histamine solution at baseline and at 1, 2, 3 ,4, 6, 8, 10 and 122 hours post-dose on days 1 and 4. The psychometrics comprised critical flicker fusion (CFF), choice reaction time (CRT), a continuous tracking task (CTT) and subjective rating scales for sedation (LARS). On days 2 and 3, subjects took their medication at pre-designated times while out of the unit. RESULTS The verum (PRM) established the sensitivity of the test battery: a significant overall reduction in CFF thresholds on both days 1 and 4 (p < 0.05); an overall significant increase (impairment) in recognition, motor and total reaction times on day 1 (p < 0.05); a significant impairment of both the tracking accuracy and reaction time aspects of the CTT task on day 1 (p < 0.005) and significantly higher ratings of subjective sedation on day 1 (p < 0.05). L-CTZ, CTZ and LOR were not distinguishable from placebo in any of the objective and subjective tests at any time point on either day 1 or day 4. With regards to the peripheral inhibitory effects, L-CTZ inhibited both the weal and flare reaction, with maximum inhibition (almost 100%) occurring within two hours of drug ingestion. CTZ also showed evidence of potent peripheral inhibition of histamine, whereas PRM, and especially LOR, showed only a weak weal and flare reaction which had completely attenuated at day 4. CONCLUSIONS In a study where the psychometric assessments were shown to be sensitive to impairment, L-CTZ 5 mg was found following both initial and repeated doses, but also to be demonstrably free from disruptive and sedative effects on objective measures of psychomotor and cognitive function. Similarly, CTZ showed evidence of pronounced antihistaminic activity and significantly reduced weal and flare scores after both acute and repeated doses, again without evidence of cognitive or psychomotor impairment. LOR also was non-sedative but the antihistaminic reaction was demonstrably weak.