The effects of age on the binding of [11C]Ro 15-4513, a partial inverse agonist of the central benzodiazepine receptor, were studied. Sixteen healthy male volunteers (21-78 years old) participated. Regional radioactivity in the brain was followed for 45 min by positron emission tomography after a bolus injection of [11C]Ro 15-4513. Similar tracer kinetics were observed in both young and old subjects. For the quantification of receptor binding in vivo, a compartment model, in which radioactivity in the pons was used as an input function, was applied. There were no significant changes in the binding potentials with age (P > 0.1) in ten brain regions. These observations delineate an interesting difference between central benzodiazepine (BZ) receptors and other neurotransmitter receptors in the human brain measured by PET that have been shown to have a reduction with age.