Besides acting in the immune system, histamine is also a neurotransmitter in the central nervous system. The H 1 R causes central side effects, e.g. of first generation antihistamines, antidepressants or antipsychotics and represents the component of the central histaminergic system most extensively studied in behavior experiments with knock-out mice. Central effects of H 2 R are similar, but only few behavioral results from knockout models are available. We summarize the behavior phenotype of H 1 R- and H 2 R-deficient mice, revealing histaminergic modulation of behaviors like locomotor activity, cognition, emotional states, arousal, sleep and wakefulness, circadian rhythm, pain perception, food intake and energy consumption, respiration and susceptibility to seizures. Knock-out models demonstrate several central effects of H 1 R and H 2 R that are not clinically and therapeutically exploited to date. We critically discuss problems and pitfalls of both the knock-out mouse technique and the pharmacological approach with receptor-selective ligands. The behavioral characterization of Hrh1 −/− - and Hrh2 −/− -mice is crucial, because many pharmacological agents lack the required selectivity to unequivocally address the functions of a single histamine receptor subtype.