A method for the preparation of

3

H-naloxone with high specific activity (57Ci/m mol) is discribed. It was found that the high-affinity binding and lowaffinity binding for

3

H-naloxone with opiate receptor have dissociation constants of

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=1.25±0.185n

M

and

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=24.5±5.05n

M

respectively. When Na was kept at 100m

М

and using

3

H-naloxone as the tracer

there was no "sodium effect". Moreover

we have designed a system having the same chemical composition but using a different tracer to study the property of an unknown endorphin. It was found that the methionineenkephalin is morphine-like and not naloxone-like.