Two groups of rats were selected from a small animal population on the basis of their exploratory activity in an elevated plus-maze model of anxiety. One group had a considerably lower and the other one a higher exploratory activity than the average total population. These subgroups were termed "anxious" and "non-anxious", respectively. In both groups central benzodiazepine binding sites in various brain structures were labelled with 3H-flunitrazepam. Peripheral benzodiazepine binding sites labelled in vitro with different tritiated ligands were also studied in several peripheral organs including blood platelets and lymphocytes. "Anxious" animals had a significantly lower number of 3H-flunitrazepam binding sites in the cerebral cortex but not in the hippocampus and cerebellum. In this subgroup 3H-Ro 5-4864 binding to peripheral benzodiazepine recognition sites was also lower than in the other one in adrenals, kidneys, platelets and lymphocytes. In the heart no differences of 3H-Ro 5-4864 binding between subgroups studied were found. Although in "anxious" rats 3H-diazepam and 3H-PK 11195 binding was significantly lower only in lymphocytes, a somewhat decreased binding to these ligands was also present in platelets. No significant differences in the affinity were found between the two groups throughout the experiments described. The results indicate that behavioral anxiety in rats is correlated not only with the lower number of central benzodiazepine receptors but also with a lower density of peripheral benzodiazepine binding sites in several peripheral organs including platelets and lymphocytes.