The study of the molecular, biochemical, genetic and cellular mechanisms of action of drugs or endogenous signaling molecules. Analysis of cellular signal transduction pathways for drugs and endogenous molecules. Study of genetic variations in drug action and drug metabolism. Specific topics in this area of study include cellular receptors, signal transduction proteins, cellular adhesion molecules, ion channels, pharmacogenetics, drug abuse, second messenger molecules, metal toxicity, oxidant toxicity and drug metabolism.