Binding of [3H]cimetidine to rat brain tissue was investigated, and a saturable binding with dissociation constant 0.22 +/- 0.05 microM found. This binding is inhibited by a range of imidazole-derived histamine H2-receptor antagonists, but not by a number of non-imidazole H2-receptor antagonists. It is concluded that the [3H]cimetidine binding site in rat brain tissue that is labelled in these experiments is not the histamine H2-receptor.