A series of 1,3-dihydro-2,1,3-benzothiadiazol-2,2-diones (I) and 3,4-dihydro-1H-2,1,3-benzothidiazin-2,2-diones (II) were prepared. While the five-member ring series (I) did not show good affinity for opioid receptors, the six-member ring series (II) exhibited extremely high affinity and selectivity for the NOP receptor and showed full agonist activity, as determined by stimulation of GTPgamma[35S] binding.