The motor effects of kassinin (KAS), substance P (SP) or neurokinin A (NKA) on rat duodenum have been investigated in vivo (urethane anesthesia) or in vitro. Intravenous neurokinins produced a phasic contraction of the duodenum that was unaffected by atropine, hexamethonium or tetrodotoxin. KAS, NKA and SP were almost equieffective in producing this phasic contraction. In addition KAS and NKA (but not SP) induced a tonic contraction with a superimposed series of rhythmic contractions. SP did not produce tonic or rhythmic contractions even in doses producing maximal effects on blood pressure. The KAS-induced tonic and rhythmic contractions were unaffected by atropine or hexamethonium and potentiated markedly by tetrodotoxin. Similar findings were obtained after topical application of neurokinins on the outer surface of the duodenum in vivo or recording the mechanical response of the longitudinal muscle in vitro. The phasic component of the KAS- or SP-induced contraction was reduced selectively, both in vivo and in vitro by [D-Pro2,D-Trp7,9]SP, an SP antagonist. It is concluded that two types of receptors mediate the motor effects of neurokinins in the rat duodenum, one of which is of the SP-P type and is mainly involved in producing the phasic contraction. The other receptor is of the SP-K type and mediates mainly the tonic contraction.