The benzodiazepine receptor ligand iomazenil was labelled with carbon-11 to allow a direct positron emission tomography/single-photon emission tomography (PET/SPET) comparison with the well-known iodine-123 labelled compound. Imaging showed the same regional distribution for both modalities. Blood sample activity was corrected for metabolites by extraction with chloroform and high-performance liquid chromatographic analysis. Metabolism is very fast: 5min after application more than 85% of the plasma activity is present as hydrophilic metabolites. Kinetic methods were used to obtain regional estimates of transport rate constants and receptor concentrations. A three-compartment model was employed which gave transport rate constants for brain uptake (K1) and the distribution volume for the specifically receptor bound compartment (DVS). K1 varied from 0.32 to 0.50ml/min per gram for the cortical regions, cerebellum, thalamus and striatum for PET and SPET. Mean DVS-PET and DVS-SPET values were, respectively, 23+/-5 and 31+/-5ml/g for the occipital cortex, 11+/-3 and 15+/-2ml/g for the cerebellum, 7+/-2 and 11+/-3ml/g for the thalamus, 5+/-3 and 10+/-3ml/g for the striatum, and 3+/-2 and 3+/-1ml/g for the pons. These values correlated very well individually. The coefficient of variation of the SPET parameters was quite comparable to that of the PET parameters, especially after 180min (PET 90min) study duration. Thus quantitative benzodiazepine receptor information can be obtained from dynamic SPET imaging in the same way as with PET.