Binding of the undecapeptide substance P to unilamellar and multilamellar lipid vesicles, made from mixtures of phosphatidylcholine and phosphatidic acid, was investigated, because liposomes may be supposed to become a useful pharmaceutical tool for the delivery of low molecular mass peptide hormones. It is demonstrated that binding is largely electrostatic in nature, and that the amount of bound peptide may be varied within rather broad limits by changing the lipid matrix and/or the aqueous medium conditions. Concerning the distribution of the peptide between inner and outer surfaces of the liposomes, large differences may be realised too, as revealed by enzymatic hydrolysis of the outside bound portions of substance P. But additionally, electrostatic binding is demonstrated also to bring about peculiar, unexpected properties of the system. These imply special relevance to a potential pharmaceutical application of substance P liposomes as well as some general relevance to in vivo application of liposomes as drug carriers.