Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a haemostatic plug. This self-amplifying mechanism requires a tight control to prevent uncontrolled platelet aggregate formation that could occlude the vessel. Endothelial-derived nitric oxide (NO) and prostacyclin (PGI₂) are strong negative regulators that modulate platelet adhesion, activation, aggregation, secretion and shape change. In this study the effects of NO on Ca²+ dependent and independent pathways of activation were investigated. The data produced during the course of this study reveals new insights into the mechanisms by which NO regulates platelet responses via the activation of the AGC family of Ser/Thr protein kinases. NO inhibited platelet shape change in a concentration dependent manner. Platelet shape change is driven by phosphorylation of myosin light chain (MLC) and the experimental data shows that NO blocked this critical phosphorylation event. Phospho-MLC generated in response to platelet agonists occurs through a Ca²+ dependent and RhoA kinase (ROCK)-dependent mechanisms and NO differentially inhibits both pathways. Activation of the ROCK pathway via RhoA leads to the phosphorylation MLC phosphatase Threonine⁶⁹⁶⁄⁸⁵³, which inhibits enzyme activity. Experimental evidence in this thesis indicates that NO, acting through cGMP and protein kinase G, prevents this inhibitory phosphorylation of MLCP by at least two mechanisms, (i) inhibiting the ROCK pathway that phosphorylates MLCP, and (ii) directly phosphorylating MLCP at an independent site, Serine⁶⁹⁵. These original observations hint at a novel mechanism for platelet regulation by the NO-cGMP-signalling pathway.