The synthesis of ten C-11-labeled aromatic and aliphatic guanidines in supercritical ammonia is described. The corresponding amines were first converted into cyanamides by reactions with [C-11]cyanogen bromide. The aliphatic amines gave high yields of C-11-labeled cyanamides (70-98%), whereas the yields for the aromatic amines was dependent on the functional groups on the ring, ranging from 0% yield for the electron-withdrawing nitro group to 90% for the electron-donating methoxy group. The conversion of the C-11-labeled cyanamides to C-11-labeled guanidines was achieved in both supercritical ammonia and aqueous ammonia solutions. The latter method gave low and irreproducible yields as compared to performing the reactions in an automated supercritical fluid synthesis (SFS) system (designed for use with supercritical ammonia). Using the SFS system, total radiochemical yields of C-11-labeled guanidines of 30-85% were obtained from the aromatic amines (depending on the substituents) and 2-36% for the aliphatic amines. A simultaneous synthesis of C-11/13-labeled phenylguanidine was performed to verify the labeling position by comparing the C-13-NMR of the labelled product to authentic reference compound.