The role of endogenous tachykinins and the mechanisms whereby they act on NK2 receptors, modulating spontaneous motility, were investigated in rat isolated proximal colon. The mechanical activity was detected as changes in intraluminal pressure. The NK2 receptor antagonist, MEN 10627, produced a concentration-dependent reduction of the contraction amplitude. [beta-Ala8]-neurokinin A(4-10), an NK2 receptor agonist, and [Sar9, Met(O2)11]-Substance P ([Sar9, Met(O2)11]-SP), an NK1 receptor agonist, induced a concentration-dependent contractile response, characterized by an increase in basal tone with superimposed phasic contractions. MEN 10627 antagonized the response to [beta-Ala8]-neurokinin A(4-10), without affecting that to [Sar9, Met(O2)11]-SP. Tetrodotoxin (TTX), hexamethonium and Nomega-nitro-L-arginine methyl ester (L-NAME) significantly reduced the response to MEN 10627. The NK3 receptor agonist, senktide, was able to activate the nitrergic inhibitory pathway, as it induced a TTX-and L-NAME-sensitive inhibitory effect. [beta-Ala8]-neurokinin A(4-10) was able to antagonize the inhibitory response to senktide. These findings suggest that tachykinins acting on NK2 receptors play a role in the modulation of the spontaneous mechanical activity. The mechanism of this action would be, in part, acting directly on the smooth muscle cells, and, in part neurogenic, sustained by nicotinic inputs, and possibly due to inhibition of NO tonic release.