It has been shown that in the cerebellum of reeler mutant mice GABA levels and GABA uptake increase while GABA binding decreases. This study shows that in the cerebellum of these mutants there is also an increase of benzodiazepine receptors. This increase is observed in cerebellar homogenates, in nuclei and in membranes. The increase in the density of central (i.e. clonazepam displacable) benzodiazepine receptors is primarily reflected in binding sites located in the GABA-receptor complex. In comparison to wild-type, GABA-modulin extracted from reeler cerebellum inhibits with a greater potency [3H]GABA binding. The increase in the central-type of benzodiazepine binding and its interaction with GABA binding, observed in cerebellar membranes, is interpreted as a functional response to the decrease in GABA binding and may reflect benzodiazepine receptor condensation and/or changes of subunit composition of the GABA/benzodiazepine receptor complex. The enhanced activity of reeler GABA-modulin reflects a functional response to the increased GABA levels in reeler cerebellum. The increase of the peripheral-type (i.e. PK 11195 displacable) of benzodiazepine receptors is probably due to metabolic changes that may accompany reeler cerebellar mutation. Differences in nuclear benzodiazepine binding between reeler and wild-type mice add a physiological importance to the nuclear binding of this drug.