In the field of medicine, nitric oxide (NO) is known for its role as a signaling molecule. One of its primary actions is to increase the levels of cyclic guanosine monophosphate (cGMP) within cells. Nitric oxide stimulates the enzyme guanylate cyclase, which converts guanosine triphosphate (GTP) into cGMP. This increase in cGMP levels leads to various physiological effects such as vasodilation, which is the widening of blood vessels. Therefore, in response to the question, "Nitric oxide acts by increasing ?", the correct answer is cGMP.