An iodinated analog of PK11195, 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)isoquinoline-3-carboxamide , a specific antagonist of the peripheral-type benzodiazepine receptor (omega 3), has been synthesized in three steps with an overall chemical yield of 40%. Both [123I]- and [125I]-Iodo-PK11195 have been synthesized by solid-state isotopic exchange in > 60% isolated radiochemical yield and specific activity of 233-348 mCi/mmol. Tissue distribution studies in rats indicate a high uptake of radioactivity in adrenal glands, heart, lung and kidneys, which was blocked 63-87% by preadministration of cold PK11195. Single photon emission computer tomography (SPECT) imaging of the canine heart has been accomplished with [123I]PK11195. These results suggest that [123I]PK11195 has potential as a SPECT radiotracer for studying the omega 3 receptor in humans.