[en] Vapreotide (RC-160), a somatostatin analog, was labeled with ^99mTc by a direct method and also by using CPTA [1,4,8,11-tetraazacyclotetradecane] as a bifunctional chelating agent. The labeled compounds were evaluated in nude mice bearing experimental human prostate cancers. In these studies, ¹¹¹In-DTPA-D-Phe-Octreotide (¹¹¹In-DTPA-octreotide) served as a standard and ^99mTc-oxytocin as a receptor-non-specific control. ^99mTc-octreotide was also used. The 24 htumor uptake of ^99mTc-RC-160 was nearly 400% higher, (p < 0.05), than that of ¹¹¹In-DTPA-octreotide and diminished upon receptor blocking. In all tissues except the kidneys, the uptake of ^99mTc-RC-160 was also higher than that of ¹¹¹In-DTPA-octreotide. The uptake of ^99mTc-RC-160 was influenced by the amount of peptide injected and the best tumor/muscle and tumor/blood ratios were obtained when only one μg of the peptide (200 Ci/mmol) was administered