Six 11C-labeled aliphatic diamines containing 4–9 linear carbon atoms were synthesized in radiochemical yields of 57% to 84% by reduction of the intermediate 11C-dinitriles prepared from Na 11CN and the corresponding bromonitrile or dibromoalkane. The biodistribution of each 11C-diamine was studied in male Sprague-Dawley rats. The prostate showed higher levels of most diamines at 30 min as compared to 5 min and 60 min after administration. The highest level of prostate activity was seen following administration of 11C-1,4-diaminobutane (putrescine). The brain and pancreas generally showed increasing concentration of activity as the aliphatic chain length increased from 6 to 9 carbon atoms. The pattern of molecular structure v. brain or pancreas tissue distribution paralleled the increasing lipophilicity associated with increasing chain length. The adrenal glands showed a progressive relative increase in tissue activity with time following the administration of 11C-1,5-diaminopentane. Most other tissues showed a progressive decrease in concentration of activity with time. Other than a possible 50% increase in putrescine concentration in the liver of rats bearing a hepatoma, the tissue distribution of 11C-putrescine in control and tumor bearing animals was similar. The hepatoma itself showed a concentration of activity 40 min after administration which was 1.82 times that in blood and 4.56 times that in muscle.