Abstract Doxepin, a tricyclic antidepressant, is one of the most potent histamine H 1 antagonists. Therefore, the binding of [ 3H]doxepin to human brain membranes was examined. Scatchard analysis revealed two distinct binding sites. The high-affinity binding site with a dissociation constant(K D ±S.E.M.)of3.1 ± 0.3 × 10 −10M was pharmacologically identified as histamine H 1 receptors. Dissociation curves at low concentrations of [ 3H]doxepin were biphasic, suggesting several possibilities about the interaction between [ 3H]doxepin and histamine H 1 receptors. Tetracyclic antidepressants, mianserin and maprotiline, were very potent, withK Ds of3.6 ± 0.7 × 10 −10M and7.9 ± 0.5 × 10 −10M, respectively. Mequitazine, a new antihistamine with a weak sedative effect, had aK Dof5.8 ± 0.8 × 10 −9, making it ten times as potent as the classic antihistamine diphenyhydramine. The highest binding of [ 3H]doxepin to histamine H 1 receptors was found in cerebral neocortex and the limbic system. The distribution of histamine H 1 receptors in human central nervous system did not correlate with the previously reported distributions in rat brain and guinea pig brain determined by [ 3H]doxepin binding.