"Peripheral" benzodiazepine binding sites (PBS) were studied in both the CNS and peripheral tissues of Maudsley reactive (MR) and Maudsley non-reactive (MNR) rats using the PBS specific ligand [3H]Ro 5-4864. A statistically significant reduction in the density of PBS was found in heart and kidney of the MR compared to the MNR. Similar reductions in the density of PBS were not observed in a number of areas of the central nervous system (including cortex, hippocampus, and hypothalamus) or other peripheral tissues, such as lung and adrenal. This selective decrease in PBS in a strain selectively bred for a high degree of "fearfulness" may be related to previous findings of a reduction in the density of PBS in the same tissues in rats subjected to uncontrollable shock. These observations suggest that PBS in heart and kidney may be altered in response to fear or anxiety.