The polyamines (putrescine, spermidine and spermine) are present in elevated concentrations in rapidly growing cell lines and in several neoplasms. Five dogs with benign or malignant tumors and one normal dog were imaged with a rectilinear scanner following I.V. administration of 2-8 mCi 11C-methylputrescine. High concentrations of 11C-methylputrescine were found in the liver, and in the kidneys and urine in the dogs with good renal function. Concentrations were higher in tumors than in skeletal muscle, blood or lung. The tumor/blood ratio varied from 2-21; tumors were demonstrated by imaging in four dogs in which the ratio exceeded 4.0. 11C-methylputrescine is a possible tumor imaging agent.