Peripheral-type benzodiazepine binding sites on intact platelets from untreated chronic insomniac patients and those chronically treated with benzodiazepine hypnotics were investigated to evaluate their putative involvement in sleep pathology and the influence of treatment. There were 34 elderly subjects in the study, 14 controls (80.7 years) and 20 insomniac patients, of whom 7 were untreated (61.1 years) and 13 were treated (84.4 years). There was an equivalent number of peripheral-type benzodiazepine 3H-PK 11195 binding sites on platelets from untreated (7.61 pmol/mg protein) and treated insomniacs (6.39 pmol/mg protein) and on platelets from the controls (6.21 pmol/mg protein). However, there was a twofold reduction in the affinity of these sites in untreated (Kd = 8.02 nM) and treated (Kd = 7.40 nM) insomniacs compared to controls (3.79 nM). This difference raises the possibility that peripheral-type benzodiazepine sites are involved in abnormal sleep.