Myosin light chain kinase (MLCK) and the kinase-related protein (KRP), also known as telokin, are the major independent protein products of the smooth muscle/non-muscle MLCK genetic locus. They share a common C-terminal part and major sites phosphorylated in vivo. Whereas MLCK is critically involved in myosin activation and contraction initiation in smooth muscle, KRP is thought to antagonize MLCK and to exert relaxation activity. Phosphorylation controls the MLCK and KRP activities. We generated two phosphorylation and site-specific antibodies to individually monitor levels of MLCK and KRP phosphorylation on critical sites. We quantified the level of KRP phosphorylation in smooth muscle before and after an increase in intracellular free Ca2+ and stimulation of adenylate cyclase, protein kinase C, and mitogen-activated protein kinases (MAP-kinases). Forskolin and phorbol-12,13-dibutyrate increased KRP phosphorylation at Ser13 from 25 to 100% but did not produce contraction in rat ileum. The level of Ser13 phosphorylation was not altered during Ca2+-dependent contraction evoked by KCl depolarization or carbachol, but subsequently increased to maximum during forskolininduced relaxation. These data suggest that several intracellular signaling pathways control phosphorylation of KRP on Ser13 in smooth muscle and thus may contribute to relaxation. In contrast, phosphorylation level of Ser19 of KRP increased only slightly (from 30 to 40-45%) and only in response to MAP-kinase activation, arguing against its regulatory function in smooth muscle.