The substance P antagonist [D-Arg 1,D-Pro 2,D-Trp 7,9,Leu 11]SP (RPTTL-SP) produced a simple competitive antagonism of the responses to substance K (SK) and substance P (SP) in a variety of tissues. In the guinea-pig ileum and field stimulated rat vas deferens the pK B values against SK were similar to those against SP indicating a single population of receptors. In the guinea-pig urinary bladder the pK B value against SK was significantly higher suggesting the existence of at least two subtypes of tachykinin receptor inthis tissue. In the hamster urinary bladder no antagonism was observed with up to 30 μM RPTTL-SP indicating the possibility of a third type of receptor.