Manipulation of intracellular signalling pathways and ligand protein interactions can lead to new therapeutic strategies in many cardiovascular diseases. One of the most commonly used methods, performed on different disease models of rat, is the microinjection of various agonist and antagonists of ligand and proteins into specific regions of the brain that are implicated in a variety of cardiovascular diseases. This chapter will describe a detailed step-by-step method for manipulation of intracellular signalling pathways in specific regions of the rat brainstem. In this method, the rostral ventrolateral medulla, an area of the brainstem which is essential for sympathetic activity and blood pressure, is studied with regard to the cyclic adenosine 3'-5'-monophosphate pathway. With this method, a more detailed interpretation of the physiological state of the rat in vivo can be achieved.