Carbachol is a synthetic muscarinic cholinergic agonist. It action is similar to that of acetylcholine, especially in the vascular smooth muscle. Its advantage is that it is stable for long periods of time (days, weeks) in aqueous solution, without losing its pharmacological action, unlike acetylcholine which hydrolyzes in solution in 30 – 45 minutes and is required to be prepared extemporaneously. Stimulating the M1-type muscarinic receptors in the vascular endothelium produces – through the G protein and the subsequent signal-transduction cascade – the activation of the inducible nitroxide-synthase. This converts L-arginine into ornithine and synthesizes nitric oxide (NO) also known as EDRF, which is the main diffusible mediator of the vascular smooth muscle relaxation. In the vascular smooth muscle, NO activates the cyclic Guanylate-cyclase GMP system, which, through the inhibition of the calcium inflows at an intracellular level, produces muscle relaxation and a decrease in the vascular tone. The administration of carbachol (10-5 M) in phenylephrine contraction plateau produces relaxation of  30-40% in average, in  the control groups of rats.