We have characterized and quantified the specific binding of [3H]-flunitrazepam ( FNZ ) to thick (230 micron) slices of mouse brain. The binding site has the characteristics of a benzodiazepine receptor, i.e., binding of FNZ is reversible, stereospecific, saturable and of high affinity. Clonazepam, but not R05 -4864, readily displaces the label. In contrast to results from homogenate assays, neither GABA nor bicuculline has any effect on [3H]- FNZ binding. However, as previously reported, the slice assay confirms the lower number of benzodiazepine receptors in "emotional" mouse brain. In addition, we have confirmed that the neurotoxin DSP4 can modify [3H]- FNZ binding though in our hands this compound elevates rather than reduces binding. The speed, simplicity and minimal tissue preparation involved suggests that this slice assay could be a valuable addition to neurochemical studies of neurotransmitter receptors.