Antagonistic peptides of prostaglandin E2 receptor subtype EP4 and their use in the treatment or prevention of medical conditions associated with oligouric nephropathy, bone resorption, abnormal intestinal crypt cell proliferation or patency of the ductus arteriosus and the like are provided herein. The antagonistic peptides of the present invention can include the following formula: X-A-R.sub.nY.sub.m wherein "X" is a hydrogen atom or an amine protecting group producing a carbamate or an amide when reacting with the amine; "A" is L-(4,4')-biphenylalanine or D-(4,4')-biphenylalanine; "R" is an amino acid selected from the group consisting of threonine, serine, tyrosine, glutamic acid, alanine, leucine and glycine; "Y" is lysine; "n" is an integer ranging from 5 to 7; and "m" is an integer ranging from 0 to 2.