The regional brain kinetics of the opioid receptor agonist, hydromorphone, labelled with 11C, were studied with positron emission tomography in two Macaque species. Rhesus and Cynomolgus monkeys. The plasma kinetics of 11C-hydromorphone was simultaneously determined. Rapid plasma elimination was obvious in both monkey species. Brain radioactivity was generally low in both, but 1.4 times higher in the Rhesus than in the Cynomolgus monkey brain. This observation might explain the more rapid development of tolerance in the Rhesus monkey.