1. Benzodiazepines are involved in the control of proliferation and differentiation of normal and malignant cells in vitro. This regulatory ability is probably mediated via peripheral benzodiazepine-binding sites. 2. In the present study we compared the binding characteristics of peripheral benzodiazepine-binding sites in human epithelial ovarian carcinoma with those in benign ovarian tumours and normal ovaries. 3. The affinity and density of peripheral benzodiazepine-binding sites in homogenate preparations of ovarian carcinoma as compared with benign ovarian tumours and with normal tissues (used as controls) were determined using a ligand specific for peripheral benzodiazepine-binding sites, [3H]PK 11195, an isoquinoline carboxamide derivative. 4. We observed a robust (three- to five-fold) increase in the neoplasm compared with benign ovarian tumours and normal tissues, without a concomitant change in affinity values. 5. This finding may reflect a change in the metabolic rates of ovarian cancer which is expressed as the alteration in the density of peripheral benzodiazepine-binding sites.