The preincubation of vascular smooth muscle cells (VSMC) with a non-peptide arginine vasopressin (AVP) antagonist (OPC-21268) (0.5 mM) for 10 min completely inhibited the 1 microM AVP-induced activation of mitogen-activated protein (MAP) kinase but did not affect the angiotensin II-, protein kinase C activator- or ionomycin-stimulated MAP kinase. Similar results were obtained with the simultaneous administration of AVP and OPC-21268. This inhibitory effect of OPC-21268 completely disappeared after washing the cells pretreated with OPC-21268. In contrast, the preincubation of VSMC with a peptide AVP antagonist, d(CH2)5Tyr(Me)AVP (1 microM), for 10 min completely blocked the 1 microM AVP-activated MAP kinase but the simultaneous administration of d(CH2)5Tyr(Me)AVP with AVP could inhibit that only by 40%. However, the washing procedure did not affect the inhibitory effect of d(CH2)5Tyr(Me)AVP. These results indicate that OPC-21268 is a specific AVP antagonist to inhibit the AVP-induced activation of MAP kinase. The antagonistic effect of OPC-21268 is also suggested to be relatively weak but prompt as compared to that of a peptide AVP antagonist d(CH2)5Tyr(Me)AVP.