Prototype inhibitors of protein arginine methyltransferases (PRMTs) have been constructed by attaching guanidine functionality via a variable linker to non-reactive amine analogues of the cellular co-factor (S)-adenosyl methionine (AdoMet). Potent inhibition of PRMT1 (IC(50) of approximately 3-6 microM) combined with weak inhibition of the lysine methyltransferase SET7 (approximately 50% of activity at 100 microM) was observed for two such compounds.