A large number of bioactive peptides have been isolated from amphibian skin secretions. These peptides have a variety of actions including antibiotic and anticancer acti-vities and the inhibition of neuronal nitric oxide synthase. We have investigated the structure–activity relationship of citropin , a broad-spectrum antibiotic and anticancer agent that also causes inhibition of neuronal nitric oxide synthase, by making a number of synthetically modified analogues.