Benzodiazepine receptor occupancy in the brain following oral administration of clonazepam (CZP) with a dose of 30 micrograms/kg in six healthy young men and a further dose of 50 micrograms/kg in one of the subjects was estimated by carbon-11 labeled Ro15-1788 and positron emission tomography (PET). The effects of CZP on the latency of auditory event-related potentials (P300) were also studied. Overall brain 11C uptake was depressed and the % inhibition of 11C uptake in the gray matter of the brain at 30 min after [11C]Ro15-1788 injection was 15.3-23.5% (mean, n = 6) following 30 micrograms/kg CZP when compared with that in the control experiment without any previous treatment. The 11C uptake in the cerebral cortex in the subject who received both doses decreased in a dose-related manner after 30 micrograms/kg and 50 micrograms/kg CZP. The P300 latency was prolonged significantly by 30 micrograms/kg CZP [31.6 +/- 16.3 ms (mean +/- SD, n = 6), P less than 0.05]. The P300 latency in the same subject was prolonged in a dose-related manner by 30 micrograms/kg and 50 micrograms/kg CZP. The technique using [11C]Ro15-1788 and PET permits comparison of the pharmacological effects with the percentage of receptor sites which benzodiazepines occupy in the human brain. P300 also seems to be useful to investigate the pharmacological effects of benzodiazepines.