Abstract We have characterized the dopamine D 2 receptor photaffinity probe, [ 3H]azido- N-methylspiperone ([ 3H]AMS). In the absence of light, [ 3H]AMS bound reversibly and with high affinity ( K d 70 pM) to sites in canine striatal membranes and was competitively inhibited by dopaminergic agonists and antagonists with an appropriate D 2 receptor specificity. Upon photolysis, [ 3H]AMS covalently incorporated into a peptide of M r 92000 as assessed by fluorography following SDS-polyacrylamide gel electrophoresis. Labelling of this peptide was specifically and stereoselectively blocked by D 2 antagonists and agonists. Minor specifically labelled peptides of M r 70000-55000 were observed under some conditions and were the result of proteolytic degradation of the peptide at M r, 92000.